True.Yep. Sephiroth's powers are almost entirely derived from Jenova; without her cells in his body, he'd be more or less a normal human.
Why Sephiroth? Because you see him there? That illusion was, as Sephiroth stated, the power of Jenova.Her powers of illusion were passed onto him. I just mentioned that bit about the Nibelheim illusion because it shows that Sephiroth can construct images in the minds of those without Jenova cells.
Jenova still controlled her cells. Jenova was still sentient. She wasn't some mindless automaton. The Reunion was so Jenova could reclaim her cells. She led Cloud there because she needed Meteor. She kept Sephiroth because she needed him to summon Meteor, and she was too physically weak to do battle.Jenova's powers lived on in Sephiroth; he was able to combine her magical abilities with the human elements of greed and pride, becoming something perhaps even more threatening. The worst of both worlds, you could say.
Sephiroth was blocking Holy for the sole reason that he's the one that summoned Meteor.
If you play through the flashback again, Sephiroth does a complete turnaround on his viewpoints. In one day, he arrives on the conclusion that everyone else is "traitors to the Cetra" and needs to be killed. That has to be Jenova's influence. No, she's not physically active, but that doesn't mean she's brain-dead. She still had full access to the illusionary powers that led to the destruction of the Cetra race. When she was dug up, after 2000 years in the ground(or however long it was), she'd have to be bloody pissed. Why would Sephiroth want to become a God? Why would Sephiroth, who was cold-hearted and calculating, want to destroy the human race? He didn't. Jenova did(she didn't want to "become a god", but she did want to destroy life).
Think about it: Sephiroth had an excuse for everything. When he destroyed Nibelheim, it was because he was killing to traitors to the Cetra. When he summoned Meteor, it was to amass the power of the Lifestream. The first reason was a falsehood(I say fed to him by Jenova), why not the second one too?
Jenova still had access to all of her cells and their powers. It'd be ludicrous is she sat back and did nothing herself. Implausible.