Back on topic...
One thing you didn't address is the final appearance of Sephiroth in the game. The last time you see him, he is in his human form. At least for me, I've always assumed that when you last face him, he was acting completely independant of Jenova and was acting on his own accord.
As far as the 'official' Square party line, anyone have any linkage for that? I've never actually read through that, I'd just like to read what they have to say on the subject.