Thank you, Sephiroth. Your response was a lot more clear and explanatory. Especially this:

Quote Originally Posted by Sephiroth View Post
He became the new JENOVA at the moment his mind got stronger and he wanted to rule all souls.
That actually jogs my memory a bit. I think I remember that from the game or one of its spin offs.

But I'm still dubious about Sephiroth "starting" Reunion. Don't the Jenova cells just do this naturally? How is that Sephiroth used Jenova to start this? Wouldn't it be whatever willpower remains in Sephiroth taking advantage of Jenova's very nature for his own goals?

Also, in the end, Jenova accomplishes her goals via Sephiroth.