Kisuke should reappear at the end of 523 if I recalled it correctly.

I don't know how you came with that conclusion... But this FF isn't that heavy.

Because Athena has more utility alive than dead. As simple as that. Medusa is not Yoruichi that Kisuke would rampage and massacre for. If someone is of use to him, he'll use him or her, even if they are his enemies. That's the reason why he was able to cooperate with the Arrancars.

One requirement to become a Captain is to have a bankai. The only shinigami who broke that rule is Zaraki.