The quote 'proving' Sephiroth's strong will seems way out of context, without more explanation I'd even be tempted to say it was a mistake in the game. Zack's strong will created Cloud, Sephiroth's contribution was his reputation, and his little hometown burning and killing spree.
A mistake in the game? Surely you can do better than that? And Zack had nothing to do with "creating" Cloud. Cloud just took most of Zack's story and put himself in it. Where does Zack's "strong will" come into it?
The game is about Jenova's reunion. It's human hero and 'villain' are both the way they are due largely to Jenova. Everything you see until Cloud goes crazy is about the reunion, the cloaked people, heading to the northern crater etc. etc. The final bosses are Jenova SYNTHESIS, then Sephiroth after joining with Jenova. If you want to claim Sephiroth actually developes the ability to turn into a giant monster, and grow wings without Jenova then well.....
Jenova's reunion is a common factor that ties many parts together, but FFVII is not "about" that. Ultimately it's about Cloud's struggle to find himself, destroy Sephiroth and settle the demons of his past. Oh, and some little thing about saving the world. And why shouldn't Sephiroth be able to change form? He was a member of SOLDIER who experienced incredible exposure to the Lifestream. If mako can turn SOLDIERs into monsters, just think what pure raw Lifestream could do to Sephiroth. Plus, y'know, he's a final boss. They ALWAYS do crazy shape changing trout.
Again I haven't played in a while, but is anyone with any real amount of dialogue near Hojo when he says it was all Sephiroth?
Everyone is near Hojo, nobody stands up and says "Nope, hojo, you're wrong!" and nobody ever does so later (regarding that matter, I mean).
Hojo injects Jenova into his body to give himself time to blow himself up so that the world can be destroyed...that's pretty crazy in my book.
Again, evil and insanity. Two different things.
If Sephiroth got into the lifestream, so did Jenova, I don't see the point.
All beings on the Planet are born from the Lifestream, and go back there when they die....except Jenova. For lack of a better word, I doubt very much she was "compatible" with it.
The lifeless blob is not only held in a thick metal tank, sealed by Shinra. It can also be diced up and still fight.
It did take most of the Cetra to take down Jenova as far as I know, and unless you hadn't noticed, by the time you fight the full Jenova, when she actually wants you dead, not only do you have some of her strength, your party is also unrealisticly strong.
But the Cetra had all these amazing powers! How can you say that thousands of them could merely trap her, when three people without Cetra powers kick her ass in ten minutes flat?
The only people ever controlled in the game have Jenova cells in them, why if it's based on strong will (despite insanity), can't other people be controlled.
Off the top of my head I can only think of one person who's directly controlled. The rest of the clones were merely manipulated and lured to the reunion as far as we know.
Cloud has both had Jenova in him, and travelled the lifestream, yet amazingly he doesn't even hint at being able to read tea leaves. Sephiroth, although admittedly more exposed developes global ESP, the ability to control super powerful ancient aliens a tiny fraction of which is the reason for his strength, and apparently the ability to mutate and grow wings.
Why must you emphasize this "omg sephiroth controlled JENOVA OMG WTF SHES POWERFUL" deal? It's just a headless blob as far as anyone else is confirmed -- until Sephiroth comes on the scene. And besides, apart from shapeshifting and impressive strength (which we're only ASSUMING she has because of the effects of her cells, btw, Jenova never actually displays that), what makes Jenova "super powerful"?
Anyway Cloud does gain knowledge, although it's unspecified. And while he's suffering from Mako poisoning, he has the ability to sense the presence of Weapon.
If mako could give you the powers 'Sephiroth' had, then you would expect at least some of the SOLDIER members to display some kind of ESP, but they don't, not even a little. Cloud doesn't either and he has both inside him.
It's not just a case of Mako exposure though, is it? Sephiroth spent five years absorbing two millenia of knowledge -- It'd be pretty worthless to mention that if there was nothing to show for it.