I'm not quoting that for the sake of us all but here's the answers.

1) Actually falling into the Lifestream helped Sephiroth cause it allowed him to increase his powers and gain mass amounts of knowledge about the Planet and its history... such as the Black Materia. The delay with his body also allowed him to stay undercover as he physically reconstructed himself and became much more powerful cause everyone assumed he had died. He had no known plan before he retrieved Jenova at the reactor, at this point of time he was still referring to Jenova as an Ancient. The Lifestream taught him about the truth behind Jenova (though he doesn't say anything about it until after your party finds out at which point he keeps up the whole. "I must avenge the Ancients!" speech) so yes, the whole plan was actually started because of Cloud's mistake.

As for the interpretation of the scene, I can't accept yours, Sephiroth is a being created to be the ultimate soldier whereas Cloud is still very much human at this point and even after going through the process to become a SOLDIER, Sephiroth is still strong enough to toy with him and smack him around like a ragdoll. Cloud is physically strong and I'm certain that adrenaline helped him overpower an injured Sephiroth, mostly cause he actually tried to fight back and was strong enough to catch him off guard. Sephiroth would expect a SOLDIER like Zack to pull that stunt off but a lowly infantrymen who can't even make it into SOLDIER or even man up and tell his friends he didn't reach his goal? The fact is, Cloud is not stronger than Sephiroth as much as he is able to take advantage of his opponents faults to eke out a slim victory. Having Cloud physically strong enough o beat Sephiroth from the beginning ruins his growth as a character and diminishes everything about SOLDIER and Sephiroth.


2) Even swinging his legs and holding the Blade Cloud would be unable to generate enough force to get him back to the ground, especially since he's stabbed in the shoulder in all version meaning his one arm is hurt and it's too high for him to gain proper leverage with his body core. He simply cannot get his body to the ground with his own strength (unless you are suggesting he's Son Goku ) unless you were going to say he caused enough shift in his weight to force Sephiroth to let him down at which point Sephiroth should not be "amazed at his power" cause all he did was shift around enough to make skewering him up that high uncomfortable for Sephiroth and forced him to let him down.