[William Cloud Hicklin]

We know two things about Tolkien as a world-builder. 1) that his thinking developed over time and he continued tinkering with his creation; as CT said "No work of my father's could truly be said to be 'finished' until it was physically taken out of his hands.'" 2) that as meticulous and convincing as Tolkien's craft was, he did make mistakes, most of which are well known to geekdom (Sam's two birth-years, anyone?)

And then we have this, from Appendix A (both editions): "There were three unions of the Eldar and the Edain: Luthien and Beren; Idril and Tuor; Arwen and Aragorn."

But what about Mithrellas?

Either Tolkien simply goofed, or he hadn't thought through his early quick comment about Imrahil, or there's a way to make it all reconcile- but again, at what stage of the mythos? It's easy enough to say Mithrellas was 'only' a Sylvan elf and thus not Eldarin, in other words an Avar, which certainly is what the Elves of Mirkwood and arguably the 'natives' of Lorien originally were; but it seems that Tolkien upgraded the Elves of Lorien and Mirkwood to Nandor (= Eldar) at a fairly early date, again arguably even before the LR depending on an obscure passage. So that's no dodge.

Unless one wants to suggest that Tolkien when he was being loose seems often to have used "Eldar" to mean "Elves of Beleriand and Aman."