One thing I would like to clear up, Sephiroth did NOT come into Shin-ra headquarters. Sephiroth did NOT take Jenova's body. And Sephiroth did NOT open Cloud's door. It was Jenova. If you pay close attention, after you come out from the cell someone (I'm think it was Barret) says that it looks like it was busted from the inside. Maybe it could have been Sephiroth's will through Jenova that caused Jenova to kill the guard and open Cloud's door, but it was not physically Sephiroth himself.