[Galin]

Well, to my mind Tolkien doesn't seem to find the matter of -- garbled texts/some measure of Noldorin interaction -- as problematic as you seem to (and others I've chatted with about this).

"What we have in the Silmarillion etc. are traditions... handed on by Men in Númenor and later in Middle-earth (Arnor and Gondor); but already far back -- from the first association of the Dúnedain and Elf-friends with the Eldar in Beleriand -- blended and confused with their own Mannish myths and cosmic ideas."

For Tolkien it starts in Beleriand!

And that said, at another point Tolkien muses about adding a note for the Wise of Numenor . . .

"The cosmogonic myths are Númenorean, blending Elven-lore with human myth and imagination. A note should say that the Wise of Númenor recorded that the making of stars was not so, nor of Sun and Moon. For Sun and stars were all older than Arda. But the placing of Arda amidst stars and under the [?guard] of the Sun was due to Manwe and Varda before the assault of Melkor."

[Christopher Tolkien comments]

"I take the words 'The Wise of Númenor recorded that the making of stars was not so, nor of Sun and Moon' to mean that the making of the Sun, Moon, and Stars was not derived from 'Elven-lore'." Christopher Tolkien, note 2 to Text I, Myths Transformed

And turning to The Drowning of Anadune, Christopher Tolkien again . . .

"Where could such ignorance of the Elves be found but in the minds of men of a later time? This I believe, is what my father was concerned to portray: a tradition of Men, through long ages become dim and confused.

"(...) I conclude therefore that the marked differences in the preliminary sketches reflect my father's shifting ideas of what the 'Mannish tradition' might be, and how to present it: he was sketching rapidly possible modes in which the memory, and the forgetfulness, of Men in Middle-earth, descendants of the Exiles of Numenor, might have transformed their earlier history."

Christopher Tolkien, commentary, The Drowning of Anadune, Sauron Defeated

I note too, that in DA Tolkien has the King of Numenor meaning to test what the Western Elves were telling his people -- that the world was round -- instead of simply taking their word for it.

"In The Drowning of Anadune the Nimir (Eldar) had come to the Adunaim an expressly taught that the world was of its nature round ('as an apple it hangeth on the branches of heaven', section 23), but Zigur coming had gainsaid it ('The World was not a closed circle', section 31). In this work the author knows that the world is of its nature a globe; but very few of the Adunaim had believed this teaching until the voyages of the survivors of the Downfall taught them it was true."

So very few of the Adunaim had believed this teaching from the Western Elves, and though it appeared to be true after the Downfall, in my opinion, it becomes easy enough to see how even a "mixed" tradition (AK) can seem to say that the World was only "made" round at the Downfall.

And although some of this commentary from the DA section refers to early-ish texts, as I've stated often enough on the web, in my opinion Tolkien "ratified" DA in the 1960s, although admittedly I base this upon a brief remark from Tolkien, and that I think DA fits perfectly into J.R.R.T.'s later characterization of The Silmarillion.