Zexion isn't one to fight, and he was quite baffled by what Axel was doing, so I'm fairly certain Replica Riku didn't have much problem killing him. So no, don't believe that one. The other one is just rediculous. I'm all for theories, have made a few myself, but those Zexion theories are coming from way out in left field and have so little validity it makes me laugh.

btw, Aeris dies and you can't revive her.