I am not qualified to answer all of those questions.
I am not sure I even understand all of the questions. They seem to indicate that you have no familiarity with the Round
cosmology idea, and yet I think you do?
Morgoth's Ring is the book to read. There different
Ainulindale versions are given, and some snips of writings were
Tolkien put down his reasoning for wanting to go Round.
I can say that in 1948 there existed Ainulindale C* (taking that CRT's dating is correct, and he does give his reaosning), which was a round version, but then he wrote Aindulindale C, which was not. Now Ainulindale C* followed B of the 30s (that given in The Lost Road and other Writings) then C followed both. C was a flat version.
So he dropped the idea for Round version... for awhile!
Then in the late fifties he went back to this, and he outlined, and even wrote some narrative, he was rewriting the myth, working in the changes that must come with the Round Earth, and early Sun.
His reason for wanting to do this was that the flat earth myth was against the scientific knowledge of the time. He decided that the flat earth version... which he called 'astronomiclly absurd' must have been of
mannish origin, since the Eldar must have known better, being in contact with the
Valar, than to think or believe that version.
In the Round earth version, the Earth was always Round, and the Sun was up before the coming of the
elves.
Iluvatar gave
varda some holy light and she gave some of this to the spirit of the Sun, Arie. Arie was then ravished by
Melkor, and he was burned and blackend, and she left Ea. But this is how the Sun was tainted in this version, rather than having come from the tree that had been touched by Melkor. Then later the Moon was made. The Two
trees were to go something like this: because of the tainted light of the Sun, there was a dome made in
Aman, with 'stars' an imitation of the night sky. At the death of the trees Aman was to be unveiled.
He also writes about the coming of
men (much earlier now) and elves and how it came to be that they joureyed west.
I am not bothered by the flat earth myth being something that 'can not have been'. I'm no astrophysicist, maybe it could!?
