[William Cloud Hicklin]

I'm not disposed to impose a rather artificial bright line of Publication as the be-all of 'canonicity,' especially since there are mistakes and inconsistencies even in the works that were published in Tolkien's lifetime.* In any event, if that is the line then it's impossible to talk about ""Nandor" or the definition of "Eldar," since all of that only appeared in print after his death. For that matter, so did Mithrellas.

It's not like Tolkien said in 1966 "OK, this is all I'm going to publish, everything else is just unreliable background stuff not to be taken seriously." He was, in his own mind anyway, trying to drive The Silmarillion towards a published form, which he died before completing. But he was stuck in a software developer's conundrum: how to upgrade the product while maintaining backwards compatability.

Where it comes to First Age matters, I would give precedence to QS/Annals materials over the LR appendices. This was, to Tolkien, THE account of the Elder Days, to which the LR just alluded in places, with some quicky sketches in the Appendices. While it's true that Tolkien in his later years had more freedom to alter the FA materials, post-66 he did surprisingly little of it; and the 2nd Ed LR was, at least in theory, altered to conform to TS as it stood at the time.

It's also the case that T was quite capable of retaining the same text while changing meaning out from under it. There is no question whatsoever that when he wrote the Lorien chapters the Elves of Lorien spoke their own Silvan language; the idea that it was just accented Sindarin was a later ret-con. (For that matter, Sindarin itself in the LR was a ret-con; the language at the time of writing was Noldorin).

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*For example, your own citation of "High Elves" was clearly a published mistake on Tolkien's part, since Luthien was not one and never was. Also the "House of Finrod" mistake which stood for years in LR printings.