I agree, Demyx has only a handful of choices: Myde, Dyme, or Edym (though of the three, I'm personally leaning towards Dyme because it's a reference to FFVII's Dyne...in a near-rhyme kind of way).
As for Luxord, I've got it (in my headcanon) narrowed it down to two choices: Rudol or Rludo (the R being silent so it's pronounced 'Ludo' which would be a neat homage to Ludo Bagman from "Harry Potter" (who was also a big gambler)).