I was surprised by the Zack-Seph thing too - it's definately implied in the game. If nothing else, I _know_ that when Cloud still thinks he Zack way at the begining, he says that he and Sephiroth were 'war buddies' or something like that XD

If you explore up in the village near the glacier, you find Gast's old house, and you can watch a bunch of flashbacks that reveal that yes, Gast is Aeris's father.

And I'm not sure if it's on that chart, as I can't read anything other than the names, or if Jen ferreted it out of one of the other sections (this thing is nearly 600 pages), but apparently Hojo and Lucrecia were both in love _and_ married. Which really cuts down the possibility of Vincent being Sephiroth's father, although I suppose it's still possible.