I've been remiss on reporting on the Japanese version. First things first: it is indeed the same disembodied voice from Prelude in Violet. That might be true in English, too, but it's far too late to trust casting consistency on that end. I rewatched the post-credits stinger from 3.0 just to be sure, and while the voices are close, it's not a match for Elidibus, and not even in the ballpark of Zenos (possessed or otherwise). My current casting ear-guess is Yuuki Kaji, which would mean the voice belongs to an entirely new character and we can safely rule out things like "older Alphinaud" etc. I can also rule out Unukalhai based entirely on pronoun ("watashi" vs Unukalhai's "boku"), just for giggles.