The opera's lyrics are in Japanese, not in Italian.
Vanilla FF6A is pretty much exactly what she should sound like in my opinion)
As far I know, all the original SNES FF6, the original FF6a and my hack are on the same pitch. If one of them is to low to fit Celes, then all 3 are. I don't understand your logic, it doesn't make a difference whether the voice comes from a real recording or a synth.
For some reason they changed the pitch of the Aria when doing it with the real orchestra, but my personal opinion is that the original SNES pitch is the "real" intended pitch from Uematsu, and as such should be preferred.
EDIT : Now that I listen to it again I can see what you see, the voice sounds unnatural, not only because the pitch, but probably because we can hear it was edited, as such it sounds worse than if a female singer actually sound at that pitch. Unfortunately, unless you can lend me an orchestra I can't do much about it
I could keep a higher pitch but then it wouldn't sound the same as the SNES... so something's going to be wrong in all cases.