You guys forgot to mention that Zack was pretty much outta it and weakened by the experiments that were being brought onto him.
Well then, if we consider Crisis Core,
how did he pull his act together in order to defeat Genesis?
One more thing - this time, about the Nibelheim incident.
Cloud remembers it as if he was Zack due to his identity mix-up. Tifa doesn't tell him that his story is all wrong because she knows something's up and she doesn't want to provoke whatever it is that's wrong with Cloud. Later, they work together to figure out what really happened, yada yada. That all makes sense. What
doesn't make sense is that Tifa at first claims that Cloud wasn't there
at all; that he didn't even witness the whole thing. But after she gets struck down by Sephiroth, Cloud tends to her and she clearly says something to him about the promise they made as children. Why doesn't she remember that? She remembers everything else perfectly well!