Well, let us not be hasty…
Or - as a more learned enthusiast once remarked:
there are Tolkien's latest thoughts, his best thoughts, and his published thoughts, and these are not necessarily the same...
Knowing how well you know your Tolkien, Inderjit,

I am sure you have also read CT's comments on the subsequent pages from where you took your first quote (HoMeX 370ff.), so I won't bother you with manifold quotes from those parts.
Of course it is up to me what I believe, but surely CT's statements on p. 383, have played a certain role that I still think that the "round earth" version represented neither Tolkien's
latest nor his
best thoughts on the issue: The one where he mentions his fathers notes where he
"...was still asking himself whether he should 'keep the old mythological story of the making of the Sun and Moon, or alter the background to a "round earth" version'..." as well as the concluding sentence:
"It may be, though I have no evidence on the question one way or the other, that he came to perceive from such experimental writing as this text that the old structure was too comprehensive, too interlocked in all its parts, indeed its roots too deep, to withstand such a devastating surgery."
I have no reason to distrust CT and refrain to believe that these mentioned notes do not actually exist. Rather, it indicates to me, that Tolkien with his post-LoTR Silmarillion was
"... pulled in two directions by competing forces: by Mythology on one side, and by History and Science on the other." as Wayne G. Hammond has put it so superbly.
Also, I can easily comprehend CT's thoughts about the "devastating" effects this surgery would have on the whole of Tolkien's Legendarium. (And not only because V. Flieger would have to re-write her book
Splintered Light 
). Light -as a symbol of "Goodness" and "Spirituality" - and Language are two crucial elements in Tolkien's Legendarium, and the Two Trees, with their nearly perfect light, deem me almost inseparably connected with Valinor and the role it plays in Arda. Summoning the awakening Quendi to the "light" of the Valar in Valinor and away from the "darkness" of Melko in Middle-earth would certainly have a different - and IMO lesser - mythological "quality", if the sun was already about at that time. Would a distinction between
Calaquendi and
Moriquendi still make the same sense and have the same "inner meaning"? These are but a few of the things that come to my mind immediately...
Hence I have no particular reason to assume that a "round earth with the sun and the moon about from the beginning" were Tolkien's
final or
definitive thoughts until someone can produce a text by Tolkien that says so...
All mythologies and Fairy-stories differ from reality, and usually not only to a small degree. Tolkien's invented mythology needed IMO not be any more realistic than other comparable mythologies which have "grown" over centuries. And which points in it one takes issue with as too "unrealistic" or "ridiculous" depends on the subjective point of view...