1. True. But I still find it hard to connect the old man's words and Zeus' personality.
2. Because he can. Maybe he just wanted a smoke, and traveled the most common way for him. :)
3. I described it to my answer to MechaRhino above. In short, when he gave you the phone to enter your phone number for him to find another traveller for the hotel, he was kind, sharp and on your side. He was even avuncular with you and Asterion about having a possible bond of more than partnership and/or friendship. Next he's showing up in the hotel during the opening night, planning something sinister against Asterion himself (according to Argos' words), and later trying to kill Argos in cold blood. I wouldn't say that this is a slow change in character, going senile gradually.
Yet again, I can be wrong, I'm more than sure that I'm wrong, I just trying to provide another viewpoint, given the clues I see. :)
edit: The one thing I'm sure of, is that Hermes' changes in personality is not a natural (for gods) things.