A) He doesn't need to see Jenova in order to be influenced by her. It's not like she's whispering things like "destroy the planet" in his ear. It's more subtle than that. She manipulates him in a way that is nearly impossible for him to identify. Like telepathically. Most likely, he thought that absolutely everything he did was his decision. I simply don't see that as being true. He made independant decisions, but they were guided by Jenova's influence.

B) If Jenova actually was meaningless, and I'm wrong about all of this, then Sephiroth is a much lamer villain than I originally thought. At least with Jenova's meddling there was some logical explanation for him going from being a hero to a murderous psychopath in less than 24 hours. If Sephiroth's transformation from hero to villain was all his doing, then that is some seriously shitty writing, dude. He throws away all notions of nobility and rationale all because he wasn't entirely human? That's just lame, and really poorly thought out. At least with the "Jenova Interference" theory he has a reasonable explanation for his actions. With the "Independant Sephiroth" theory, all he's doing is throwing a giant temper tantrum for no good reason.