: : I don't see it that way. First of all, Elwing was there for quite some time and she was in no way below Gil-galad. Second, Elrond was of dual heritage. Through his father Ealendil and his mother Idril he was a Noldo and would certainly accept Gil-galad as high king of the Noldor.

[I think that is what I said, but I will rephrase]
Elwing was at the Havens of Sirion, Gil-galad was on Balar. At this point it is unlikely that Gil-galad was recognized as High King of the Elves of the West. He was likely just High King of the Noldor. It is possible that Cirdan saw him as his overlord.
Elwing fled the destruction Havens while Elrond was quite young. Elrond was taken by Maglor and raised by him. It is implied that Elrond participated in the War of Wrath (he seems to have been at the breaking of Thangorodrim). As Maglor did not participate in the War of Wrath, it can be inferred the Elrond left his care.
Elwing was the leader of the survivors of the Ruin of Doriath, after she departed this would presumably have fallen onto Elrond. Of course he was unavailable. But after he left Maglor it is possible that he had contact with the Sindar who knew him for the descendant of Thingol. He would also have had contact with Gil-galad, his rightful lord through his father side. If he recognized Gil-galad as his King, and the Sindar saw Elrond as their lord through blood right it may have been easy for them to accept Gil-galad as High King of the Eldar (in the Second Age). Those who did not (eg Oropher and Amdir Malgalad) passed east and founded their own realms.

: : That is an entirely different thing than saying he - or the Sindar in general - would accept Gil-galad as high King of all the Eldar in Middle-earth. The Sindar were not too happy with Noldor of any stripe after the sack of Doriath by the Feanoreans.

Not true. The Noldor of Gondolin (of whom Idril and Earendil were leaders) and the Sindar of Doriath got on quite well. As a matter of fact there is quite a famous marriage between two of them. Earendil, leader of the Noldor (and Sindar) of Gondolin, and Elwing, who led the survivors of Doriath, seemed to get along quite well (unless Earendil's voyages were because he could not stand her;)). The Sindar of the Falas who fled to Balar with Cirdan, and the Noldor of Hithlum and Nargothrond who fled to Balar and put themselves under Gil-gald seemed to get along well enough. Presumably there was intercourse between Balar and the Havens as well.
Also nothing was said about Gil-gald being 'High King of All the Eldar of Middle-Earth'. Just those of 'the West'. Presumably this was a nominal title, like that of Ingwe, in any event.

: : That sort of dual status we see in Elrond was quite common in medieval Europe where you could end up with some quite odd vassalage relationships. Elrond's would be similar - on one level (as a Noldo) he would be beneath Gil-galad on another level (as scion of Thingol) he would be Gil-galad's equal.

: : It would be quite a big step for the Sindar to "acknowledge" Gil-galad as high king as Tolkien reports in the Appendix. It is my opinion that it is just this sensitive political situation that caused Gil-galad to name Celeborn and not Elrond as his vice-regent of South Lindon which had a primarily Sindarin population. It would be politically "correct" for Elrond, as Noldo, to remain with Gil-galad as his herald. It would be quite another thing for Gil-galad to be perceived as appointing Thingol's "heir" to rule over Sindar.

The other reason could be that Celeborn was already there. He and Galadriel had passed of the Ered Luin before the end of the First Age. Gil-galad was Galadriel's King in any event and she assuredly had some influence in South Lindon. Later in the Second Age Gil-galad appointed Elrond as his vice-regent in Eriador in any event. Elrond prefered to reckon his lineage through Elwing to Thingol by the way.

: Which brings up the interesting question: Why didn't Elrond simply claim the Sindarian crown for himself? He obviously had the right to do so. Had Elrond been King of all the Sindar then perhaps Sauron would have been prevented from making The One.
: -Thentaran

Because he did not want it. Perhaps he realized that it would be better to be unified under Gil-galad. The Sindar at this point (begining of Second Age) were probably few and scattered. If Elrond had been King of the Sindar how would that have prevented Sauron from making the One?