Lets see here...I guess I'll just follow your order and make corrections as I go.
1. After the initial battle in the reactor inside the Nibelheim mountains, Cloud and Zack where taken back to the Shinra Mansion where professor Hojo conducted a series of scientific experiments on them. The experiments had no affect on Zack, not true for Cloud. Cloud suffered memory loss and distortion. This is why Cloud had his stories mixed up. I strongly doubt the were treated differently during the experiments, after all their pods were right next to each other.
2. SOLDIER wasn't just made up of Zack and Sephiroth. SOLDIER was the elite force, and in it you had the top performers, Zack and Sephy. Cloud was in SOLDIER, just not first class. But still pretty damn high on the chart since he was selected to go on the exhibition to Mnt. Nibelheim.
3. Lets talk about the 'three victories over Sephiroth...
a) The first fight was unfair in sense of 1 versus 1. Sephiroth had already dueled it out with Zack - But if you want to talk fair, Cloud hadn't been exposed to Mako energy and injected with cells from Jenova/Jenova project. So now we have Cloud - man, versus wounded, yet super-human Sephiroth.
b) During the series of second fights (if you want to look at it like that), the only assistance Cloud received while facing Sephiroth was against Bizarro Sephiroth, and the One-Winged Angel...also known as Safer Sephiroth. The clash against the shirtless Sephiroth was used convey the mental grasp he held over Cloud's mind, mind control if you will. Remember after they defeat Safer Sephiroth, Cloud is the only one who can still feel his presence. This fight, I would say, is the fairest of them all. Cloud equiped with his Buster Sword (no materia)...And Sephiroth with his masamune (no materia). But again, it was all mental. This is explained through the storyline of the game.
c) I will concur that Cloud may have been outmatched while going against Sephiroth for the final time. But that does not eliminate all chance of victory. Sephiroth got cocky. And he started to taunt Cloud - he made it personal. And simply put, it pissed Cloud off. His limit had been broken...
LIMITBREAK!
...of course Sephiroth wasn't trying. He was playing with his food. And it bit him on the ass. Duh.
4. Cloud and Tifa aren't lovers. Just best friends. Final Fantasy 7 isn't a love story. I think you have him mixed up with this douche
...god that game sucked.
5. You must be joking about Darrel...
You can go ahead and hate on Cloud all you want. The fact that he was the loser / underdog, and yet was the only one who managed to physically defeat Sephiroth makes him a hero. You can't deny that.
So stfu and gtfo you fucking noob