I've postulated before that the wasteland of TWL may have been Eliot's
lack of creativity (or his belief in it as expressed in a letter
or two.) Part of "What the Thunder Said" would be his celebration
of the return of his creativity.
If I were take your assumption that Eliot's "flash of lightning" came
from the Paradiso I would say that the allusion would be to the flash
(fulgore) in Canto 33. Here we had Dante comparing his flash of
insight to that needed to see how the circle could be squared.
Perhaps Eliot saw this as analogous to his writer's block and when the
inspiration for "What the Thunder Said" came to him (and possibly also
how all the pieces of the poem could fit together) he decided to
include this flash into the poem.
Seen this way we still the lightning coming from Paradiso, and in
perhaps a spiritual way and we can have Verdenal too if we want. But
Verdenal would not be in the flash of inspiration, he would be part of
the sterility of the author's life and creativity that the inspiration