Thanks for the response Aiwendil, and you put things well (arguably better than I did above) as to what I'm trying to suggest.
If memory serves, Christopher Tolkien notes some of the intended confusions woven into the mannish account, but does not include 'what the Elves taught' concerning the shape of the World as one of them. One might argue that his references were never intended as a complete list in any case, but still, it would seem very odd to me if Men also got
this wrong.
Again there are plenty of later references that the Silmarillion is now to be imagined as a largely Mannish affair -- thus it had incorporated Mannish ideas; and to my mind, the very basic notion is that the Elves from Aman should better know the 'truth' about the World than Men, even some of the West-men. Still, this seems to me a way to preserve the early ideas Tolkien wondered about in
Myths Transformed (seen as a collective set of musings) -- a way to keep them alive, or part of the written legendarium, without need of major revision.
What is 'more true' within the mixed account, or
Akallabeth? Hard to say, but we can at least compare AK (mixed) to DA (mannish), and within DA I think we have the seed of a theoretically updated FN account (Elvish) with respect to the shape of the World -- unless for some reason what the Elves of the West taught in DA runs quite contrary to the Elvish account...
... or some other reason others might think of! which is why I'm tossing this out for consideration