[Maédhros]

The note where he is the son of Fingon has him sent in 456, after the Dagor Bragollach. The question arises: if he is the son of Orodreth, when and how did he come to the havens? Was it still in 456? Was it later? Somehow he escaped the Battle of Tumhalad - and unless we intend to embark on some major creative rewriting of the Narn, he was not there at all during Turin's stay. That puts the date at 490 at the very latest. Of course, we will have to say at some point in the narrative that Gil-Galad was sent to the havens, so we will apparently have to decide upon some definite date - but there is nothing to suggest any specific time, with the only precedent the old date of 456.

I don't see a contradiction or a the need for a rewrite of the Narn with the fact that Gil-Galad being in Nargothrond when Túrin arrives. If Gil-Galad were not to be mentioned in the Narn, it does not means that he was not there and that he didn't escaped as the note in the Parentage of Gil-Galad suggests. The date could well be 495.
The note states:

Finrod left his wife in Valinor and had no children in exile. Angrod's son was Artaresto, who was beloved by Finrod and escaped when Angrod was slain, and dwelt with Finrod. Finrod made him his 'steward' and he succeeded him in Nargothrond. His Sindarin name was Rodreth (altered to Orodreth because of his love of the mountains .. ..... His children were Finduilas and Artanáro = Rodnor later called Gil-galad. (Their mother was a Sindarin lady of the North. She called her son Gil-galad.) Rodnor Gil-galad escaped and eventually came to Sirion's Mouth and was King of the Ñoldor there.

I think that the notes seems to imply that Gil-Galad would have escaped the Fall of Nargothrond and eventually came to the Mouths of Sirion.

But Círdan went to Balar in 473, following an attack by Morgoth not long after the Nirnaeth. If Gil-Galad was with him at the havens before that time, he surely would have gone withe Círdan to Balar. If he was not, then it's hard to imagine why Orodreth should send him to the havens later on, and not to join Círdan on Balar.

If we follow the note of the Parentage that Gil-Galad escaped the Fall of Nargothrond, this is not a problem.

Then there is the reference to his "eventually" becoming "King of the Noldor at the mouths of Sirion". Note first of all that this is in a note that is primarily concerned with his parentage and name, not with the speficics of his movements. In such a context "the Mouths of Sirion" could easily be a loose way of referring to the whole of the havens, the delta, and the Isle of Balar. Keep in mind that Círdan is said to have maintained a foothold on the mainland even after removing to Balar. "King of the Noldor" could then be taken at face value - he inherited the title that had passed from Fingolfin to Fingon to Turgon.

The fact that Círdan had kept a foothold in the mainland can and does indeed explain how they were able to send their help (late) against the sons of Fëanor.

How would we arrange it that Gil-Galad was not on Balar yet? We would have to invent some story whereby he either came to the havens after 473 or for some reason remained there after Cirdan. Either way, we would seem to have to take too much liberty.

I really don't see the liberty that you are talking about Aiwendil. We have the fact that the 456 travel of Gil-Galad to the Havens was rejected and we have a note by JRRT stating that Gil-Galad escaped and went to the Havens. Correct me if I'm wrong but I think that when JRRT says escaped, he means after the Fall of Nargothrond. Why is it a liberty if we are following a note by JRRT?
Also we have to remember that the Narn was written ca. 1950, while the Shibboleth was written ca. 1968.

I think it's safer to interpret the note more broadly, and retain both 456 as the date of his departure for the havens and the story that he went to Balar with Cirdan in 473.

I have to disagree with this. We can of course use more broadly the Sirions Mouth statement but what about the escaped part. Are we simply going to ignore that fact?
Is there is a principle that goes against using the late not of JRRT?

About the Ingwiel/Ingwion/Finarfin passage: I don't think that there's any matter of the story in doubt here. As I understand it, the situation is this: in QS, it is said that Ingwiel was the chief of the host, but the wording was such that it appears that he is named the chief of the Noldor. In LQ, he corrected this by changing Ingwiel to Finarfin, so that it correctly names the chief of the Noldor. In the typescript, he apparently did not observe the error in wording, but made the unrelated correction Ingwiel > Ingwion. "Light-elves" was changed to "Fair-elves here because it refers specifically to the Vanyar.

I'm not sure. While it is most probable that it was a slip, we have to see that the typescript is certainly the later version of it. If JRRT did indeed read that part and did not add Finarfin as the leader of the host of the Noldor, then it could be that he could have changed his mind.