How the rest will be divided up (my theory)

[JJ Strife]

I can't remember the exact specifics of what's supposed to be going on in the original. Is the version of Sephiroth throughout the first disc supposed to be just Jenova's headless body pretending to be Sephiroth?

Or is it another clone person just carrying the body? Like is suggested in Remake.

Your point could be true, but the player doesn't understand any of the reunion stuff at that point. So I think it does need a practical rationalization for the story to make sense.
But then again, they've already kinda abandoned the idea of a self-contained story by randomly including Zack, even though newcomers have no idea who he was.