Jenova Rebirth: Interesting theory, but, as Pheonix says, it doesn't really explain much. You chasing two Sephiroths isn't the most believable theory, but I'll think about it.
Big D: As Pheonix said, I don't believe Sephiroth controlled Jenova at all. Rather, if I'm arguing for the support of the manifestation theory, then I'd say he absorbed some of the power from the mako he was encased in for five years. Remember "Sephiroth's" plan? To absorb mako/Lifestream energy at its source. Maybe he could've absorbed some of the power from such a prolonged exposure. That also helps to explain how he was able to summon Meteor.
