1- olympus might not exist anymore, with the lack of worshippers it just couldn't keep functionig as a home for the gods.
2- why would hermes even bother to do that? seems like too elaborate a scheme to pretend to be someone else when PC wouldn't have anyway of knowing anything by that point.
3- it really wasn't that fast, the gods have been degrading for a long time, hermes is just starting to show more severe symptoms, but he was certainly much weaker than in his prime when the story begins. don't let his composure fool you.