[Findegil]

I did also forget to put one of the most important sources into the collection, the UT version of the later Tale of Tuor (TO). It does not reach the conversation between Tuor and Turgon, but we have the encounter of Tuor with Ulmo and we have some other interesting hints:

‘Arise, Tuor, son of Huor!’ said Ulmo. ‘Fear not my wrath, though long have I called to thee unheard; and setting out at last thou hast tarried on thy journey hither. In the Spring thou shouldst have stood here; but now a fell winter cometh soon from the land of the Enemy. Haste thou must learn, and the pleasant road that I designed for thee must be changed. For my counsels have been scorned, and a great evil creeps upon the Valley of Sirion, and already a host of foes is come between thee and thy goal.’
‘What then is my goal, Lord?’ said Tuor.
‘That which thy heart hath ever sought,’ answered Ulmo: ‘to find Turgon, and look upon the hidden city. For thou art arrayed thus to be my messenger, even in the arms which long ago I decreed for thee. Yet now thou must under shadow pass through peril. Wrap thyself therefore in this cloak, and cast it never aside, until thou come to thy journey's end.’
Then it seemed to Tuor that Ulmo parted his grey mantle, and cast to him a lappet, and as it fell about him it was for him a great cloak wherein he might wrap himself over all, from head to foot.
‘Thus thou shall walk under my shadow,’ said Ulmo. ‘But tarry no more; for in the lands of Anar and in the fires of Melkor it will not endure. Wilt thou take up my errand?’
‘I will, Lord,’ said Tuor.
‘Then I will set words in thy mouth to say unto Turgon,’ said Ulmo. ‘But first I will teach thee, and some things thou shall hear which no Man else hath heard, nay, not even the mighty among the Eldar.’ And Ulmo spoke to Tuor of Valinor and its darkening, and the Exile of the Noldor, and the Doom of Mandos, and the hiding of the Blessed Realm. ‘But behold!’ said he, ‘in the armour of Fate (as the Children of Earth name it) there is ever a rift, and in the walls of Doom a breach, until the full-making, which ye call the End. So it shall be while I endure, a secret voice that gainsayeth, and a light where darkness was decreed. Therefore, though in the days of this darkness I seem to oppose the will of my brethren, the Lords of the West, that is my part among them, to which I was appointed ere the making of the World. Yet Doom is strong, and the shadow of the Enemy lengthens; and I am diminished, until in Middle-earth I am become now no more than a secret whisper. The waters that run westward wither, and their springs are poisoned, and my power withdraws from the land; for Elves and Men grow blind and deaf to me because of the might of Melkor. And now the Curse of Mandos hastens to its fulfilment, and all the works of the Noldor shall perish, and every hope which they build shall crumble. The last hope alone is left, the hope that they have not looked for and have not prepared. And that hope lieth in thee; for so I have chosen.’
‘Then shall Turgon not stand against Morgoth, as all the Eldar yet hope?’ said Tuor. ‘And what wouldst thou of me, Lord, if I come now to Turgon? For though I am indeed willing to do as my father and stand by that king in his need, yet of little avail shall I be, a mortal man alone, among so many and so valiant of the High Folk of the West.‘
‘If I choose to send thee, Tuor son of Huor, then believe not that thy one sword is not worth the sending. For the valour of the Edain the Elves shall ever remember as the ages lengthen, marvelling that they gave life so freely of which they had on earth so little. But it is not for thy valour only that I send thee, but to bring into the world a hope beyond thy sight, and a light that shall pierce the darkness.’
And as Ulmo said these things the mutter of the storm rose to a great cry, and the wind mounted, and the sky grew black; and the mantle of the Lord of Waters streamed out like a flying cloud. ‘Go now,’ said Ulmo, ‘lest the Sea devour thee! For Ossë obeys the will of Mandos, and he is wroth, being a servant of the Doom.’
‘As thou commandest,’ said Tuor. ‘But if I escape the Doom, what words shall I say unto Turgon?’
‘If thou come to him,’ answered Ulmo, ‘then the words shall arise in thy mind, and thy mouth shall speak as I would. Speak and fear not! And thereafter do as thy heart and valour lead thee. Hold fast to my mantle, for thus shalt thou be guarded. And I will send one to thee out of the wrath of Ossë, and thus shalt thou be guided: yea, the last mariner of the last ship that shall seek into the West until the rising of the Star. Go now back to the land!’
Then there was a noise of thunder, and lightning flared over the sea; and Tuor beheld Ulmo standing among the waves as a tower of silver flickering with darting flames; and he cried against the wind:
‘I go, Lord! Yet now my heart yearneth rather to the Sea.’
...
‘I do not bid you to lead me further than the gate,’ said Tuor. ‘There Doom shall strive with the Counsel of Ulmo. And if Turgon will not receive me, then my errand will be ended, and Doom shall prevail. But as for my right to seek Turgon: I am Tuor son of Huor and kin to Húrin, whose names Turgon will not forget. And I seek also by the command of Ulmo. Will Turgon forget that which he spoke to him of old: Remember that the last hope of the Noldor cometh from the Sea? Or again: When peril is nigh one shall come from Nevrast to warn thee? I am he that should come, and I am arrayed thus in the gear that was prepared for me.’
Tuor marvelled to hear himself speak so, for the words of Ulmo to Turgon at his going from Nevrast were not known to him before, nor to any save the Hidden People. Therefore the more amazed was Voronwë; but he turned away, and looked toward the Sea, and he sighed.
...
Elemmakil saluted him and said: ‘Here have I brought Voronwë Aranwion, returning from Balar; and here is the stranger that he has led hither, who demands to see the King.’
Then Ecthelion turned to Tuor, but he drew his cloak about him and stood silent, facing him; and it seemed to Voronwë that a mist mantled Tuor and his stature was increased, so that the peak of his high hood over-topped the helm of the Elf-lord, as it were the crest of a grey sea-wave riding to the land. But Ecthelion bent his bright glance upon Tuor, and after a silence he spoke gravely, saying: ‘You have come to the Last Gate. Know then that no stranger who passes it shall ever go out again, save by the door of death.’
‘Speak not ill-boding! If the messenger of the Lord of Waters go by that door, then all those who dwell here will follow him. Lord of the Fountains, hinder not the messenger of the Lord of Waters!’
Then Voronwë and all those who stood near looked again in wonder at Tuor, marvelling at his words and voice. And to Voronwë it seemed as if he heard a great voice, but as of one who called from afar off. But to Tuor it seemed that he listened to himself speaking, as if another spoke with his mouth.
For a while Ecthelion stood silent, looking at Tuor, and slowly awe filled his face, as if in the grey shadow of Tuor's cloak he saw visions from far away. Then he bowed, and went to the fence and laid hands upon it, and gates opened inward on either side of the pillar of the Crown. . Then Tuor passed through, and coming to a high sward that looked out over the valley beyond, he beheld a vision of Gondolin amid the white snow. And so entranced was he that for long he could look at nothing else; for he saw before him at last the vision of his desire out of dreams of longing.
Thus he stood and spoke no word. Silent upon either hand stood a host of the army of Gondolin; all of the seven kinds of the Seven Gates were there represented; but their captains and chieftains were upon horses, white and grey. Then even as they gazed on Tuor in wonder, his cloak fell down, and he stood there before them in the mighty livery of Nevrast. And many were there who had seen Turgon himself set these things upon the wall behind the High Seat of Vinyamar.
Then Ecthelion said at last: ‘Now no further proof is needed; and even the name he claims as son of Huor matters less than this clear truth, that he comes from Ulmo himself.’

The text is closely temporary to the GA and we see here only slight differences in wording between the warnings of Ulmo to Turgon and Tuor quoting them.

I will try to group and summarize the sources:
LT:
Tuor ask Turgon first to prepare for war.
When Turgon denies, Tuor urges him 'dare greatly' which only can mean to start that war and describes the terrible battle and glorious outcome if Turgon would follow the wishes of Ulmo.
When Turgon denies again, Tuor ask him to send messengers into the west to move the Valar to a war against Melkor.
Turgon denies again by reason of the former fruitless trials.

Sketch and Q I:
Tuor ask Turgon to prepare for the war since Ulmo will move the Valar to send him succour. He describes the terrible battle and glorious outcome if Turgon would follow the wishes of Ulmo. If Turgon denies he is to ask him depart with his people from Gondolin to the mouth of Sirion seeking back to Valinor by the help of Ulmo.
Q II is very similar to Q I, but we have two great differences. The first is the role of Tuor, if Turgon would do the biding of Ulmo. In the Sketch and Q I he is to go with a force to Hithlum and bring the Men of Hithlum back to an aliance with Turgon. In Q II it is to the East that he should go for the same reason and supposedly without a force of warriors.
The second is more interesting here. It is a new element in Q II, were Ulmo does suggest a reconciliation with the Feanorians. And in the same text we get for the first time Turgons vow after the second kin-slaying never to make an alliance with the Feanorians. These two elements (Ulmo asking for an alliance and Turgon later definite denial) are most clearly connected.

GA, TO and ToY:
The message was not given in full, but the wording does for me suggest that it was more than just ask Turgon to abandon Gondolin and go down to the sea. In TO Ulmo reveals to Tuor that 'now the Curse of Mandos hastens to its fulfilment, and all the works of the Noldor shall perish, and every hope which they build shall crumble.' Even as Tuor does this could be read to mean that 'Turgon [shall] not stand against Morgoth, as all the Eldar yet hope'. But the answer of Ulmo is not straightforward. He does rather suggest that Tuors 'sword' would be crucial to the outcome of such a battle.
The mentioning of the denial of Trugon ever to build an alliance with the Feanorians in ToY C does, for me, suggest that such biding of Ulmo was at that time still present. And for what could such an alliance be good, if not for a war against Morgoth? That it is not mentioned in ToY D does not matter much to me, since it is true for many elements of the story that survived for sure. I think ToY was a kind of a working chronology for Tolkien. While writing down the different versions he put into them elements that at these moments seem important to him. In that way it is much more telling that an element is mentioned, than the missing of it in the next version.
But one part of the message is clearly gone and that we did so far not eliminate in our text (even so gondowe suggested to do so): Ulmo addresses of Voronwe in TO as 'the last mariner of the last ship that shall seek into the West until the rising of the Star.' Thus Tuor can not ask Turgon to send messengers into the west, because Ulmo foretold already that it would be worthless.
On the other hand it seems clear that Tuor has to ask Turgon (at least at the end of the conversation) to leave Gondolin, which we also so far did not put into our text.

Before I start to build a text for this section we better would find a common ground what elements we will take up. My conclusions from the text study are:
Tuor ask Turgon to prepare for a War against Morgoth (I would leave it open if he is to start that war or if he should only prepare for the war to come, no version of the text did clearly urge Turgon to an assault, even so LT hardly could be interpreted other than in that way).
Turgon denies.
Tuor speaks about the means (alliance with the Men of the east and the Feanorians, probably succour by the Valar urged to this by Ulmo) and consequences of that war (terrible battle but a chance for a real victory).
Turgon denies again.
Tuor bides him to abandon Gondolin and search refuge under Ulmos protection at Sirions mouth (he mentions unknown dangers greater than expected and a harder future for Middle-Earth if that course is taken).
Maeglin speaks against the counsel of Ulmo and Idril supports it, so she and the wiser councilors are already troubled that Turgon did not follow the original biding.
Turgon denies again by blaming the Valar for being blind and deaf against his messengers asking for help and of being not helpful so far in protecting his people against Morgoth.

Respectfuly
Findegil