Personaly I like to think that Sephiroth is Projecting Himself outwords from the cave or is actuly there in the flesh. The cave being to most likely truth. I allso think that it is Sephiroth that is Controling Janova, i belive this becouse he controls all the Clones as well as Cloud. Somehow the Combanation used to creat Sephiroth made him more powerfull then the being(Janova) Whos Cells he was infushed with. THis would explane why He still has so much power even after you slay janova in the north cave. He seems to allso be able to creat janova clones as it were since you fight it meny times in the game and each time it's a new boss completly. I think i even remember seeing him drop a chunk of Janova when ever you fight one. So i belive Sephiroth is indeed to mastermind behind it all. In fact Janova might not have been one of the Cetra at all. Areis was Cetra Janova is some kind of.....being.