Well, as we've seen so far, Peter needs time to figure out how to use other peoples' abilities, plus proximity is a factor as well. Seeing as how Sylar kept his distance right up until they fell, I don't see how Peter would have been able to combat him. Plus, Sylar has obviously mastered his gift, while Peter using his power for the first time would be more like " hey! I can bend this spoon with my mind"
I think your theory on Zach = :crazy:
edit: and what regeneration did Sylar portray? None. He landed on Peter. That's probably how he survived.