[Findegil]

gondowe, if I understood you correctly, you see Ulmos message only as a kind of trigger for Tuor to reach Gondolin. The real reason behind it is only to allow Tuor to father Earendil.

Thinking in that way, meaning considering the reasoning of divine beings, is not easy. Ulmo is not Eru. He, as all the Valar has a different reception of time and history in comparison to us but he is not omniscient. We know that the Valar could in their mind move backward and forward in time. So it seems obvious that being inside the history of Arda the reception of the Valar of events laying in the future seen from the moment of reception, was restricted to their knowledge of the Music of the Ainur and the vision of Arda shown to them by Eru. Eru in contrast is omniscient, he would be able to know all the events even to the most minute point at any given time of history for any given time of history (the same is true for the Ainur that stayed with Eru, since their existence always was outside time, while the Valar by entering Arda at the beginning of time ‘bound’ themselves to history and with that to time). Thus Ulmo could say ‘the days of Release draw nigh’ but I doubt that he could pinpoint it to a definite date.
In the same way I would doubt that he could presage the exact outcome of the War of Wrath before it happened. I think that he did know that the might of Melkor or better the influence of the Melkor ingredient in the History of Arda would be diminished greatly. But I also do not doubt that he did know that this was not a war to end war and that there would be two further ages of Arda ending in a war against an agent dealing with the Melkor ingredient. But it seems clear that none did know that this would be Sauron (otherwise why should Eonwe allow Sauron stay in Middle-Earth?) or by which means (in part with Orks and other minions of Melkor left behind) Sauron could work.

So what does it all mean for the special case at hand?
I agree that Ulmo did probably know that the most important part of Tuors journey was the fathering of Earendil (Ulmo at Vinyamar: ’… even from Nivrost one shall come to warn thee, and from him beyond ruin and fire hope shall be born for Elves and Men.’). But still Ulmo says to Turgon in Vinyamar ‘But love it[Gondolin] not too well, and remember that the true hope of the Noldor lieth in the West and cometh from the Sea.’ If only bringing forth Earendil was the sole propose, why then warn Turgon in that way? For me it seems clear that had Turgon followed Ulmos biding brought to him by Tuor the (end-)result most have been better then it turned out to be.
Now we could speculate a lot what that bidding could have been, and how that might have changed the history, but why? We have the old versions and nothing newer. In the old versions Trugon is bidden to gather all the free people of Middle-Earth to his banner and start the War. Ulmo does not promise him that he would outlive that War; he does only promise that the victory in that War would be more effective then the result of the War of Wrath in the event was.

May be the best way to bring the project forward in this arcane situation is start from scrap: What sources do we have? And what do they say? Let’s go through:

Lost Tales: There is first a passage describing Ulmos speech to Tuor, then a long passage of active speech of both Tuor and Turgon:

… Then Ulmo arose and spake to him and for dread he came near to death, for the depth of the voice of Ulmo is of the uttermost depth: even as deep as his eyes which are the deepest of all things. And Ulmo said: "O Tuor of the lonely heart, I will not that thou dwell for ever in fair places of birds and flowers; nor would I lead thee through this pleasant land, o but that so it must be. But fare now on thy destined journey and tarry not, for far from hence is thy weird set. Now must thou seek through the lands for the city of the folk called Gondothlim or the dwellers in stone, and the Noldoli shall escort thee thither in secret for fear of the spies of Melko. Words I will set to your mouth there, and there you shall abide awhile. Yet maybe thy life shall turn again to the mighty waters; and of a surety a child shall come of thee than whom no man shall know more of the uttermost deeps, be it of the sea or of the firmament of heaven." Then spake Ulmo also to Tuor some of his design and desire, but thereof Tuor understood little at that time and feared greatly.
…
Then Turgon king of Gondolin robed in white with a belt of gold, and a coronet of garnets was upon his head, stood before his doors and spake from the head of the white stairs that led thereto. "Welcome, O Man of the Land of Shadows. Lo! Thy coming was set in our books of wisdom, and it has been writtenthat there would come to pass many great things in the homes of the Gondothlim whenso thou faredst hither." Then spake Tuor, and Ulmo set power in his heart and majesty in his voice. "Behold, O father of the City of Stone, I am bidden by him who maketh deep music in the Abyss, and who knoweth the mind of Elves and Men, to say unto thee that the days of Release draw nigh. There have come to the ears of Ulmo whispers of your dwelling and your hill of vigilance against the evil of Melko, and he is glad: but his heart is wroth and the hearts of the Valar are angered who sit in the mountains of Valinor and look upon the world from the peak of Taniquetil, seeing the sorrow of the thraldom of the Noldoli and the wanderings of Men; for Melko ringeth them in the Land of Shadows beyond hills of iron. Therefore have I been brought by a secret way to bid you number your hosts and prepare for battle, for the time is ripe."
Then spake Turgon: "That will I not do, though it be the words of Ulmo and all the Valar. I will not adventure this my people against the terror of the Orcs, nor emperil my city against the fire of Melko."
Then spake Tuor: "Nay, if thou dost not now dare greatly then will the Orcs dwell for ever and possess in the end most of the mountains of the Earth, and cease not to trouble both Elves and Men, even though by other means the Valar contrive hereafter to release the Noldoli; but if thou trust now to the Valar, though terrible the encounter, then shall the Orcs fall, and Melko's power be minished to a little thing."
But Turgon said that he was king of Gondolin and no will should force him against his counsel to emperil the dear labour of long ages gone; but Tuor said, for thus was he bidden by Ulmo who had feared the reluctance of Turgon: "Then am I bidden to say that men of the Gondothlim repair swiftly and secretly down the river Sirion to the sea, and there build them boats and go seek back to Valinor: lo! the paths thereto are forgotten and the highways faded from the world, and the seas and mountains are about it, yet still dwell there the Elves on the hill of Kor and the Gods sit in Valinor, though their mirth is minished for sorrow and fear of Melko, and they hide their land and weave about it inaccessible magic that no evil come to its shores. Yet still might thy messengers win there and turn their hearts that they rise in wrath and smite Melko, and destroy the Hells of Iron that he has wrought beneath the Mountains of Darkness."
Then said Turgon: "Every year at the lifting of winter have messengers repaired swiftly and by stealth down the river that is called Sirion to the coasts of the Great Sea, and there builded them boats whereto have swans and gulls been harnessed or the strong wings of the wind, and these have sought back beyond the moon and sun to Valinor; but the paths thereto are forgotten and the highways faded from the world, and the seas and mountains are about it, and they that sit within in mirth reck little of the dread of Melko or the sorrow of the world, but hide their land and weave about it inaccessible magic, that no tidings of evil come ever to their ears. Nay, enough of my people have for years untold gone out to the wide waters never to return, but have perished in the deep places or wander now lost in the shadows that have no paths; and at the coming of next year no more shall fare to the sea, but rather will we trust to ourselves and our city for the warding off of Melko; and thereto have the Valar been of scant help aforetime."
Then Tuor's heart was heavy, and Voronwe wept; ...

In this account we do have the presaged of Tuor coming ‘in the books of wisdom’, but we are not told by whom nor when. What is also missing is any hint of the second kin-slaying and Turgons reaction thereto.

In a prose fragment of the Tale of Tuor written after LT [HoME IV; chapter 1;i] it does not tell of the message but of Ulmos motives:

'Then' said Ilfiniol son of Bronweg 'know that Ulmo Lord of Waters forgot never the sorrows of the Elfin kindreds beneath the power of Melko, but he might do little because of the anger of the other Gods who shut their hearts against the race of the Gnomes, and dwelt behind the veiled hills of Valinor heedless of the Outer World, so deep was their ruth and regret for the death of the Two Trees. Nor did any save Ulmo only dread the power of Melko that wrought ruin and sorrow over all the Earth; but Ulmo desired that Valinor should gather all its might to quench his evil ere it be too late, and him seemed that both purposes might perchance be achieved if messengers from the Gnomes should win to Valinor and plead for pardon and for pity upon the Earth; for the love of Palurien and Orome her son for those wide realms did but slumber still. Yet hard and evil was the road from the Outer Earth to Valinor, and the Gods themselves had meshed the ways with magic and veiled the encircling hills. Thus did Ulmo seek unceasingly to stir the Gnomes to send messengers unto Valinor, but Melko was cunning and very deep in wisdom, and unsleeping was his wariness in all things that touched the Elfin kindreds, and their messengers overcame not the perils and temptations of that longest and most evil of all roads, and many that dared to set forth were lost for ever.
Now tells the tale how Ulmo despaired that any of the Elfin race should surpass the dangers of the way, and of the deepest and the latest design that he then fashioned, and of those things which came of it.

In the Sketch of the mythology we have both Ulmos presaging and Ulmos message in full and Turgons refusal and so the second kin-slaying is told we have no reaction of Tugon to it.

The people of Turgon escaping aided by the prowess of Hurin were lost from the knowledge of Morgoth, and indeed of all in the world save Ylmir. In a secret place in the hills their scouts climbing to the tops discovered a broad valley entirely encircled by the hills in rings ever lower as they came towards the centre. Amid this ring was a wide land without hills, except for one rocky hill that stuck up from the plain, not right at the centre, but nearest to that part of the outer wall which marched close to the edge of Sirion.'
Ylmir's messages come up Sirion bidding them take refuge in this valley, and teaching them spells of enchantment to place upon all the hills about, to keep off foes and spies. He foretells that their fortress shall stand longest of all the refuges of the Elves against Morgoth, and like Doriath never be overthrown - save by treachery from within. …
...
... He [Tuor] is to bid Turgon prepare for battle against Morgoth; for Ylmir will turn the hearts of the Valar to forgive the Gnomes and send them succour. If Turgon will do this, the battle will be terrible, but the race of Orcs will perish and will not in after ages trouble Elves and Men. If not, the people of Gondolin are to prepare for flight to Sirion's mouth, where Ylmir will aid them to build a fleet and guide them back to Valinor. If Turgon does Ylmir's will Tuor is to abide a while in Gondolin and then go back to Hithlum with a force of Gnomes and draw Men once more into alliance with the Elves, for 'without Men the Elves shall not prevail against the Orcs and Balrogs'. This Ylmir does because he knows that ere seven full years are passed the doom of Gondolin will come through Meglin.
...
… Turgon is grown old and very mighty and proud, and Gondolin so fair and beautiful, and its people so proud of it and confident in its secret and impregnable strength, that the king and most of the people do not wish to trouble about the Gnomes and Elves without, or care for Men, nor do they long any more for Valinor. Meglin approving, the king rejects Tuor's message in spite of the words of Idril the far-sighted (also called Idril Silverfoot, because she loved to walk barefoot) his daughter, and the wiser of his counsellors. …

Q30 does repeat the same in nearly the same words. But here for the first time is given a reaction of Turgon to the second kin-slaying. I give first the earlier version QI:

… In this [the building of Gondolin] Turgon had the aid of the messages of Ulmo, that came now up the river Sirion; for his voice is to be heard in many waters, and some of the Gnomes had yet the lore to harken. In those days Ulmo was filled with pity for the exiled Elves in their need, and in the ruin that had now almost overwhelmed them. He foretold that the fortress of Gondolin should stand, longest of all the refuges of the Elves against the might of Morgoth,' and like Doriath never be overthrown save by treachery from within. …
…
… But now Ulmo bade him [Tuor] make all speed to Gondolin, and gave him guidance for the finding of the hidden door; and words were set in his mouth to bear to Turgon, bidding him prepare for battle with Morgoth ere all was lost, and promising that Ulmo would win the hearts of the Valar to send him succour. That would be a mortal and a terrible strife, yet if Turgon would dare it, Morgoth's power should be broken and his servants perish and never after trouble the world. But if Turgon would not go forth to this war, then he must abandon Gondolin and lead his people down Sirion, ere Morgoth could oppose him, and at Sirion's mouth Ulmo would befriend him, and lend his aid to the building of a mighty fleet wherein the Gnomes should sail back at last to Valinor, but then grievous would be the fate of the Outer Lands. Tuor's part if Turgon should accept the counsels of Ulmo, would be to go forth when Turgon marched to war and lead a force into Hithlum and draw its Men once more, into alliance with the Elves, for 'without Men the Elves shall not prevail against the Orcs and Balrogs'.
This errand did Ulmo himself perform out of his love of Elves and of the Gnomes, and because he knew that ere twelve years were passed the doom of Gondolin would come, strong though it seemed, if its people sat still behind their walls.
…
Tuor spoke his embassy to Turgon in the great square of Gondolin before the steps of his palace; but the king was grown proud and Gondolin so fair and beautiful and he was so trustful of its secret and impregnable strength, that he and the most of his folk wished no longer to trouble with the Gnomes and Men without, nor did they long more to return to the lands of the Gods. Meglin spake against Tuor in the councils of the king, and Turgon rejected the bidding of Ulmo, and neither did he go forth to war nor seek to fly to the mouths of Sirion; but there were some of his wiser counsellors who were filled with disquiet, and the king's daughter spake ever for Tuor. She was named Idril, one of the fairest of the maidens of the Elves of old, and folk called her Celebrindal, Silverfoot, for the whiteness of her slender feet, and she walked and danced ever unshod.
…
… They succoured not Nargothrond or Doriath, and the wandering Elves knew not how to find them; and when Turgon learned of the slaying of Dior, he vowed never to march with any son of Feanor, and closed his realm, forbidding any of his folk to go ever forth.

In the second Version the first part about Ulmos help in building the city is the same. The rest reads as follow in QII:

But now Ulmo bade him [Tuor] make all speed to Gondolin, and gave him guidance for the finding of the hidden door; and a message he gave him to bear from Ulmo, friend of Elves, unto Turgon, bidding him to prepare for war, and battle with Morgoth ere all was lost; and to send again his messengers into the West. Summons too should he send into the East and gather, if he might, Men (who were now multiplying and spreading on the earth) unto his banners; and for that task Tuor was most fit. 'Forget,' counselled Ulmo, 'the treachery of Uldor the accursed, and remember Hurin; far without mortal Men the Elves shall not prevail against the Balrogs and the Orcs.' Nor should the feud with the sons of Feanor be left unhealed; for this should be the last gathering of the hope of the Gnomes, when every sword should count. A terrible and mortal strife he foretold, but victory if Turgon would dare it, the breaking of Morgoth's power, and the healing of feuds, and friendship between Men and Elves, whereof the greatest good should come into the world, and the servants of Morgoth trouble it no more. But if Turgon would not go forth to this war, then he should abandon Gondolin and lead his people down Sirion, and build thee his fleets and seek back to Valinor and the mercy of the Gods. But in this counsel there was danger more dire than in the other, though so it might not seem; and grievous thereafter would be the fate of the Outer Lands.
This errand Ulmo performed out of his love of the Elves, and because he knew that ere many years were passed the doom of Gondolin would come, if its people sat still behind its walls; not thus should anything of joy or beauty in the world be preserved from Morgoth's malice.
… There Tuor spake the embassy of Ulmo, and something of the power and majesty of the Lord of Waters his voice had caught, so that all folk looked in wonder on him, and doubted that this were a Man of mortal race as he declared. But proud was Turgon become, and Gondolin as beautiful as a memory of Tun, and he trusted in its secret and impregnable strength; so that he and the most part of his folk wished not to imperil it nor leave it, and they desired not to mingle in the woes of Elves and Men without; nor did they any longer desire to return through dread and danger to the West. Meglin spoke ever against Tuor in the councils of the king, and his words seemed the more weighty in that they went with Turgon's heart. Wherefore Turgon rejected the bidding of Ulmo; though some there were of his wisest counsellors who were filled with disquiet. Wise-hearted even beyond the measure of the daughters of Elfinesse was the daughter of the king, and she spoke ever for Tuor, though it did not avail, and her heart was heavy. Very fair and tall was she, well nigh of warrior's stature, and her hair was a fountain of gold. Idril was she named, and called Celebrindal, Silver-foot, for the whiteness of her foot; and she walked and danced ever unshod in the white ways and green lawns of Gondolin.
…
… Tidings Turgon heard of Thorndor concerning the slaying of Dior, Thingol's heir, and thereafter he shut his ear to word of the woes without; and he vowed to march never at the side of any son of Feanor; and his folk he forbade ever to pass the leaguer of the hills.

The next text to be looked upon are the Earliest Annals of Beleriand (AB I the later Version AB II does not reach the Fall of Gondolin). In them the founding of Gondolin is recorded much earlier and Turgon does find Tumladin because he is trouble by dreams send by Ulmo. The Presage of Tuor coming is not in this Annals and the encounter of Tuor with Ulmo and Turgon is very condensed:

Ulmo himself appears to him in Nan-tathrin; and Tuor and Bronweg guided by Ulmo find Gondolin. They are received after questioning, and Tuor speaks the embassy of Ulmo. Turgon does not now harken to it, partly because of the urging of Meglin. But Tuor for his kindred's sake is held in great honour.

What followed were the Later Annals of Beleriand (AB2) based closely on AB II as far as it goes and then on AB I. Therefore again the presage of Tuors journey is not included. And the passage of Tuor meeting with Ulmo and Turgon very short:

Ulmo himself appeared to Tuor in Nantathrin, and Tuor went thence up Sirion, and guided by Ulmo found the entrance to Gondolin. There Tuor spake the embassy of Ulmo; but Turgon would not now harken to it, and Meglin urged him to this against Tuor.

In neither of the Annals up to this point did we have a recount of teh reaction of Trugon to the second kin-slaying.

The Quenta Simarillion (QS) version written between 1930 and 1937 never reached the Fall of Gondolin. And even so the departure of Turgon to Gondolin is reported the message of Ulmo upon his departure is not.

This message is reported in the Gray Annals (GA) under the year 116 and later under year 496 are also reported the meetings of Tuor with Ulmo and Turgon, the second kin-slaying is not reached by this text:

In this year Gondolin was full-wrought, after fifty [added: and 2] years of secret toil. Now therefore Turgon prepared to depart from Nivrost, and leave his fair halls in Vinyamar beneath Mount Taras; and then [for the last time Ulmo himself came to him >] Ulmo came to him a second time / and said: 'Now thou shalt go at last to Gondolin, Turgon; and I will set my power in the Vale of Sirion, so that none shall mark thy going, nor shall any find there the hidden entrance to thy land against thy will. Longest of all the realms of the Eldalie shall Gondolin stand against Melkor. But love it not too well, and remember that the true hope of the Noldor lieth in the West and cometh from the Sea.'
And Ulmo warned Turgon that he also lay under the Doom of Mandos, which Ulmo had no power to remove. 'Thus it may come to pass,' he said, 'that the curse of the Noldor shall find thee too ere the end, and treason shall awake within thy walls. Then shall they be in peril of fire. But if this peril draweth nigh, then even from Nivrost one shall come to warn thee, and from him beyond ruin and fire hope shall be born for Elves and Men. Leave, therefore, in this house arms and a sword, that in years to come he may find them, and thus shalt thou know him and be not deceived.' And Ulmo showed to Turgon of what kind and stature should be the mail and helm and sword that he left behind.
…
Here Tuor son of Huor met Bronwe of the Noldor at the mouths of Sirion; and they began a journey northward along the great river. But as they dwelt in Nan Tathrin, and delayed because of the peace and beauty of that country in the spring, Ulmo himself came up Sirion and appeared to Tuor, and the yearning for the Great Sea was ever after in his heart. But now at Ulmo's command he went up Sirion, and by the power that Ulmo set upon them Tuor and Bronwe found the guarded entrance to Gondolin. There Tuor was brought before King Turgon, and spake the words that Ulmo had set in his mouth, bidding him depart and abandon the fair and mighty city that he had built, and go down to the Sea. But Turgon would not listen to this counsel; and [Meglin later >] Glindur his sister-son spoke against Tuor. But Tuor was held in honour in Gondolin, for his kindred's sake.
This annal was much emended and added to (and the date changed to 495), and then (since the text was now in a very confused state) struck out as far as 'bidding him depart' and replaced by the following version on a detached slip:
495 Now Tuor Huor's son had lived as an outlaw in the caves of Androth above Mithrim for four years, and he had done great hurt to the Easterlings, and Lorgan set a price upon his head. But Ulmo, who had chosen him as the instrument of his designs, caused him to go by secret ways out of the land of Dorlomin, so that his going was hidden from all the servants of Morgoth; and he came to Nivrost. But there, becoming enamoured of the Sea, he tarried long; and in the autumn of the year Ulmo himself appeared to Tuor, and bade him to depart, and go to the hidden city of Turgon. And he sent to him Voronwe, last of the mariners of Turgon, to guide him; and Voronwe led Tuor eastward along the eaves of Eryd Wethion to Ivrin. (And there they saw Turin pass, but spoke not with him.) And at the last by the power that Ulmo set upon them they came to the guarded gate of Gondolin. There Tuor was brought before the king, and spoke the counsel of Ulmo, bidding Turgon [the following is the text already given] depart and abandon the fair and mighty city that he had built, and go down to the Sea. But Turgon would not listen to this counsel; and [Meglin later >] Glindur his sister-son spoke against Tuor. But Tuor was held in honour in Gondolin, for his kindred's sake.

The Later Quenta Silmarillion did add nothing to the GA entrance for 116 in which the last meeting of Ulmo and Turgon in Nevrast is told and never reached the Fall of Gondolin.

There remains one text of quite a different kind. A plot synopsis for the story of Turin given in HoME 11 in the Chapter The Wanderings of Húrin but it does not tell much:

Tuor escapes from Hithlum by Cirith Ninniach and comes to Nivrost. He meets Gelmir and Arminas. Ulmo visits him on the shores by Mount Taras, and sends Voronwe to him. Tuor and Voronwe go to seek Gondolin which they reach in winter.

Last but not least as we have already mentioned that text several times in the discussion we have the Tale of the Years (ToY). It has several stages of development (A to D) which I will try to give in full for the important parts. None of excerpts that we were given cover the founding of Gondolin nor the coming of Tuor to the city. Of importance is rather the second kin-slaying and the reaction of Turgon to it.
ToY A:

506 The Second Kin-slaying.
507 The Fall of Gondolin. Death of King Turgon.

ToY B:

509 (Spring) Second Kinslaying. Last warning of Ulmo to Gondolin.
510 The fall of Gondolin at Midsummer. Death of King Turgon.

So no report of Turgons reaction in this early phases.

ToY C:

511 [> 509] The Second Kinslaying. The Sons of Feanor assail[ed] Dior, and he was slain; slain also were Celegorm and Curufin and Cranthir. Eldun and Elrun sons of Dior were left in the woods to starve. Elwing escaped and came with the Silmaril to the Mouths of Sirion. Ulmo sends a last warning to Gondolin, which now alone is left; but Turgon will have no alliance with any after the kinslaying of Doriath. Maeglin Eol's son, sister-son of Turgon, was taken in the hills, and betrayed Gondolin to Morgoth.
512 [> 510] The Fall of Gondolin. Death of King Turgon.

Luckily the last warning of Ulmo from ToY B is gone, but therefore we get a reaction of Turgon to the assault of the Feanorians upon Dior. The interesting thing is that it has not been in the earlier stages. So it is clear that Tolkien did not copy it dully as has been argued for other such cases. He rather remembered it (probably by rereading of Q30) and put it actively into ToY C.

ToY D:

506-507 At Yule Dior fought the sons of Feanor on the east marches of Doriath, and was slain. There fell also Celegorn (by Dior's hand) and Curufin and Cranthir. The cruel servants of Celegorn seize Dior's sons (Elrun and Eldun) and leave them to starve in the forest. (Nothing certain is known of their fate, but some say that the birds succoured them, and led them to Ossir.) [In margin: Maidros repenting seeks unavailingly for the children of Dior.] The Lady Lindis escaped with Elwing, and came hardly to Ossir, with the Necklace and the Jewel. Thence hearing the rumour she fled to the Havens of Sirion.
509 Maeglin captured by spies of Melkor (Sauron?).
510 Midsummer. Assault and sack of Gondolin, owing to treachery of Maeglin who revealed where it lay.

So here the reaction of Turgon to the second kin-slaying is again left out.

I hope that I cached all relevant sources. My own conclusions have to wait another day until I find time to go over all the sources collected here again.

Respectfuly
Findegil