fascistcrusader
Restricted
For starter's its Genesis, not Gackt. Second of all the original scene in Nibelheim cuts away right before Genesis shows up, and doesn't start up again until days later, so the addition of Genesis is not in contradiction with anything from the original.
Then we have the fact that Sephiroth is already going nuts before Genesis shows up. He is already starting to question his existence exactly like he did in the original, the only difference is instead of not showing us what happened, it shows us Genesis was there betweeen the reactor and the library.
Crisis Core shows Sephiroth's descent just as the original UOG describes it. It was being so close to Jenova that allowed her instincts to overcome Sephiroth while he was in a weaker state of mind, and caused him to destroy Nibelheim and go nuts. It was only after Sephiroth fell into the lifestream he realized the truth of Jenova and used his powerful will to take over its mind and made it his slave.