Really? By her hair colour I thought she was related to Amros somewhow
Here's my twenty cents on the letter anyway.
'the promise made to the Eldar (High Elves – not to other varieties, they had long ago made their irrevocable choice, preferring Middle-earth to paradise) they would always be able to leave Middle-earth and pass on the Straight Road.' 1954
In 1955 Tolkien published [Appendices] that the Silvan Elves of Lórien and Mirkwood were mostly East-elves, and not Eldar [but not necessarily Avari, as Tolkien does not employ this term in the final, author-published account].
'The Elves far back in the Elder Days became divided into two main branches: the West-elves (the Eldar) and the East-elves. Of the latter kind were most of the elven-folk of Mirkwood and Lórien; but their languages do not appear in this history, of which all of the Elvish names and words are of Eldarin form.* (footnote) *In Lórien at this period Sindarin was spoken, though with an 'accent', since most of its folk were of Silvan origin (...) But Lórien, Caras Galadhon, Amroth, Nimrodel are probably of Silvan origin, adapted to Sindarin.'
And in another quote of note from Appendix F the word
Eldar is said to refer to the three kindreds who sought for the Undying Realm
'... and came there are the beginning of days, save the Sindar only' Meaning (as I take it) the Eldar are those Elves who passed Oversea, plus the Sindar only. Basically the West-elves, with the Sindar making it to Beleriand, a very 'Western land' of course, in ancient days.
Here the term Eldar is basically equivalent to 'West-Elves', contrasted to East-elves. I know that's not how the 1977 Silmarillion 'defines' Eldar, but so far I think even JRRT would have to admit that he, at the least strongly implied that the Silvan Elves are not Eldar nor their languages Eldarin.
So when Tolkien says 'not to other varieties' might he mean various clans of Avari? Avari means 'refusers' which works, and we know there were different clans of Avari at least, but does 'variety' really work in this context?
The other term is 'High Elves'. In the letter Tolkien seems to equate it with Eldar. I think that is not in itself an error, even though the more usual application of High Elves, in Middle-earth anyway, refers to the Noldor. But if High Elves equals Eldar here, as
is suggested, then to my mind JRRT has at least implied other varieties that are 'not Eldar' have made an irrevocable choice, which again, according to The Lord of the Rings at least, could possibly include both Avari and the Silvan Elves of Lórien and Mirkwood. Or East Elves.
But on the other hand Tolkien can hardly mean the Silvan Elves in general, as The Lord of the Rings tells the story of Nimrodel [Lothlórien] and Silvan Elves that fled Lórien to sail Over Sea [Appendix B].
Hmm
The 1977 Silmarillion 'defines' Eldar as all those who took up the March, even if they did not pass Over Sea or make it to Beleriand. In other words, all those who did not initially 'refuse' the Summons, it seems. The Silvan Elves of Mirkwood and Lórien would be 'Eldar' by this description, and perhaps notably in a late text Tolkien refers to the Tawarwaith 'Wood Elves' as 'Eldar in origin' [Unfinished Tales].
In another late text Amroth basically tells Nimrodel that it's okay for those who took up the Great March to still pass over Sea. He is Sindarin, she is a Silvan Elf. Amroth tells Nimrodel that it is said: '... the grace the Valar gave to us to pass over the Sea is granted also now to any of those who made the Great Journey, even if they did not come in ages past to the Shores and have not yet beheld the Blessed Land.'
Toss in a 'definition' of Eldar [at least one of them] employed by Christopher Tolkien for The Children of Húrin, and we seem to have returned to the idea of The Lord of the Rings! Meaning Eldar refers to those Elves who took up the Great March Over Sea plus the Sindar [West Elves]. 'Eldar The Elves of the Great Journey out of the East to Beleriand.' 'To Beleriand' is interesting here: I would say beyond Beleriand surely, but at least to Beleriand would once again suggest the Sindar only, 'only' of those Eldar who did not actually pass Over Sea.
Toss in Quendi And Eldar where Tolkien mixes in some Avari in the Anduin Vale, then toss in later notes from Unfinished Tales where the Tawarwaith are simply said to be Telerin in origin, hardly to be distinguished from Avari...
... and it's all very simple
My conclusion so far: Jackson invented Tauriel, gave her the red brown hair of Nerdanel's kin, and possibly made her an Avar to 'create something' in a possible 'love story' of some measure, even if only hinted at, or a 'love story' in potential.
And if she's an Avar Jackson fans can [if they like] point to Quendi And Eldar for Avari arguably in Mirkwood, or employ the argument that the term East Elves doesn't necessarily exclude the Avari.
And what Tolkien meant in this letter I have no idea