I agree that any of the heroes from last season not recognizing each other is weird, since they all met in the last ep. Heck, even if Nathan doesn't remember a particular face since he was focused on Claire and Peter, everyone else at least should remember the guy who flew up with his brother in the sky to save New York.
Was also wondering what was going on with Kensei at the end. The two possibilities I've come up with have already been mentioned - either Hiro time-shifted Kensei's body back in time, like Dragonblade mentioned, or Kensei himself is manifesting regenerative powers similar to Claire's.
I didn't think it stupid of Claire to cut off her toe. Yeah, she knows she can come back from death, from being cut open for autopsy, from being burned by radiation - but she didn't know whether or not a part of her body that was wholly missing would grow back.
EDIT for Richards; As I recall, her fingers were maimed after that incident, but not outright cut off.