You haven't proven anything, you've given two vague quotes that don't say anything about Sephiroth developing powers that aren't just unexplained they're unrealistic even for a fantasy game.
You've explained Sephiroth using his own image? How strong a magic do you need to mutate something on the other side of the planet into a duplicate of a human? I doubt that anyone would go to all that trouble just because it's how they happen to look. If you have a remote control toy, going to the effort of taking a photo of yourself and sticking it on the front so it looks like you seems like a lot of effort for no reason, look at what your claiming Sephiroth does.
The game never says that it's Jenova in control, it never says Sephiroth is in control undeniably, Jenova is just a hell of a lot more likely.
It's not about Sephiroth summoning Meteor unless you believe it is, if you don't Jenova isn't taken out quickly at all. Actually even if you believe Sephiroth's in control she still isn't taken out quickly, unless you believe he has the stupid amount of powers you think he has.
You only actually see Sephiroth twice after he goes crazy, and he only speaks on one of those occasions. I don't see why you are so quick to dismiss Jenova.
