Well said, but you can't assume that gives him psychic powers. He only mentions that he has obtained knowledge in the game.--- Temple of the Ancients
He says he is far greater than the Cetra, and the only evidence he gives of that is k"knowledge". Materia is condensed life energy. Thats why you can use magic of the Cetra. But Sephiroth was in the Life Stream, which consists of natural, uncondensed Life energy. There is no evidence that one can use the powers of the planet just by being in the life strem for a long time. That is only an assumption. It is an even more unlikely assumption even to the "assumption" that Jenova is greater than you say it is.
But in FFVII, knowledge is power, quite literally:
Sephiroth
"You were in SOLDIER and didn't even know that?"
"...the knowledge and wisdom of the Ancients is held in the
materia."
"Anyone with this knowledge can freely use the powers of the Land and the Planet. That knowledge interacts between ourselves and the planet, calling up magic..... or so they say."
How can he absorb the energy of the Life Stream so easily? A strong will? I think he most likely had some help from a greater being--- Jenova.
Think of Cloud - when he arrived at Mideel the doctor said he's had an immense amount of Mako-drenched knowledge put in all at once. Cloud pretty much caved in and contracted Mako poisoning. But Sephiroth's stronger than that. I think, given five years, he could quite certainly make use of all that "knowledge" he comes across. And we know he did, because he suddenly knows all about the Black Materia and Meteor (and the fact that he flat out tells us he gained the knowledge). There's never any indication that Jenova's involved somehow.

Sephiroth did what he did because of what Hojo and Jenova did to him, so there's two other parties to blame without even thinking.
Hojo "created" Sephiroth in a sense, but that doesn't make him responsible for his son's actions. Once Sephiroth discovered the truth of his origins, he made the choice to kill. Jenova didn't "do" anything to him, no blame can be placed on her really.
The reunion is a natural process? What on earth is that meant to mean? Are you posting specifically to get someone banned for flaming you?
Never heard of natural process? Take my hand, as an example. If I draw a blade across the palm, the skin breaks and I bleed. Given time, though, the blot clots and my skin begins to heal, until it's no longer broken. It's back together. I didn't use any special powers to do this, it's a completely natural healing process. Even's Hojo's wording is suggestive of this. He plain says "You see, even if Jenova's body is dismembered, it will eventually become one again. That's what is meant by Jenova's Reunion."

"it will eventually become one again"? This doesn't suggest a conscious decision at all, sounds more natural IMO.
Only one person is seen to be directly controlled? Are you playing the game or reading a script from somewhere? LOADS of people are seen to be controlled.
One person is seen to be directly controlled -- Cloud. The Clones are led by Sephiroth, who's presumably the one filling their heads with all that "master" crap, but I don't recall seeing Sephiroth ever literally controlling them in the way he does with Cloud.
Sephiroth AND Jenova are in the crater, Cloud has personal issues with Sephiroth as that is he is the one he saw burn down his hometown, and the image that killed Aeris.
Uh huh, and Jenova gets no mention. She's like a semi-forgotten hurdle by the time we reach her in the Crater. She's there, but easily jumped over and quickly forgotten in favor of bigger hurdles.
Jenova was in the lifestream with Sephiroth, so really that doesn't make any sense. You again failed to mention strong will.
Why doesn't it make sense? Perhaps Jenova couldn't wield the true power of the lifestream (her job is pretty much consuming it, remember?). I wouldn't be surprised if she couldn't do a damn thing other than eat it, considering she wasn't even partly made from FFVII's planet.
Why are you waiting for dialogue? We've TOLD you we aren't going to give you any, stop asking for it.
Because dialogue is the only feasible proof when it comes to a theory this contrary to the game's own story. The fact that you can't provide any real evidence is pretty bad news for your theory.