Elrond and Elros' children

Thanks, I was too lazy to look it up. Did you get that from HoME XII?

By definition the Eldar are ANY of the Quendi who accepted the Summons and went on the Great March, they did not have to complete it. The Nandor are Eldar by definition. The Tawarwaith are comprised of Nandor (who are Nelyarin (Lindarin) Eldar) and Nelyarin (Lindarin) Avari, who are not Eldar, as well as Sindar (who are Eldar). It is not stated whether mixed blood descendants of the two groups woud be considered Eldar or Avari. It is mentioned that some few Avari who came to Beleriand and intermingled with the Sindar were accepted as Eldar. See 'Quendi and Eldar' in War of the Jewels.

Thanks for a reference. I've just started reading WotJ, so I haven't gotten that far yet.

I have The Peoples of Middle-earth thankfully (got it recently), but that's not where the info is in. You'll find it in Unfinished Tales in the Appendices of Of Galadriel and Celeborn. I don't remember which Appendix, but it's called I think something along the lines of "Of the Silvan Elves" and/or "The Sindarin Princes of the Silvan Elves". Somewhere in that area.

You're quite welcome. I have The Peoples of Middle-earth thankfully (got it recently), but that's not where the info is in. You'll find it in Unfinished Tales in the Appendices of Of Galadriel and Celeborn. I don't remember which Appendix, but it's called I think something along the lines of "Of the Silvan Elves" and/or "The Sindarin Princes of the Silvan Elves". Somewhere in that area.

Tar, although the Sindar of the Woodland Realm can be classified as Wood-elves, I don't think the Silvan's actual name, the Tarwarwaith reffers to them. I could be wrong.

I don't think you are wrong. I should have clarified my intent a bit more. My post at that point was poorly worded (but I think you understood my intent). Though it could be argued that a variety of Sindar who chose to dwell among and become part of the Nandorin and Avarin inhabitants of the Woodland Realms (seeking a lesser or more natural life), in such a manner as to become hadly distinguishable from them could be considered Tawarwaith (Forest-folk).

Eruve: I think the 3 Unions quote may have come from such a time. For a while the Nandor were considered Pereldar (Half Eldar) (at this point I think only the Calaquendi and Sindar were Eldar which may be what was being refered to earlier) but I will need to look into it more.
But even then it is still not entirely consistant. It does not address Dior (who by the judgement of Manwe is mortal) and Nimloth (Lindis, Elulil) his wife (one could argue that as a Peredhel he is not being considered of the Edain in that manner (especially if pointing out that Elrond and Celebrian do not count). But then also Arwen was not really an Elda. She was Peredhil (and what of her brothers, did they stay and wed mortals? Unknown but if 'only 3 Unions' must be taken as 'gospel' then no). It is just not entirely consistant (that is not to say that a fit could not be made, but I dont mind the possible contradiction).

The problem with defining Elven kindreds is that Tolkien changed things around after The Lord of the Rings was published. There were no Nandor in 1954-5 when the book first came out. There were the Eldar, the West-elves, and the East-elves, the Wood-elves. The Eldar included the Sindar and the Noldor, the latter being the High Elves, and (then) unnamed other kindreds which remained in Aman.

So Imrahil's Elvish ancestry was Silvan in origin, and his ancestry therefore did not conflict with the statement that there were three unions of the Eldar and the Edain.

Gwindor's words as reported in The Silmarillion are just one Elf's guess and opinion, nothing more.