I would say, that it is not. Before the notorious scene with P, Argos clearly says that there is something wrong with Hermes, he's planning something bad, and even sent Argos to the hotel. After that Hermes is trying to kill Argos in cold blood with amount of witnesses around. So either he's a good in pretending (because he was seemed mostly as a nice and decent character before these events), which is hardly in line with the idea, that he's already senile. Senile people aren't good at playing elaborate roles of good persons, while planning something bad. Or he's really already senile, and that is happened sometimes between he left the hotel in his full mind (after you gave him your phone number) and before he returned. And that is a really short time span of weeks or even days.
I hope I convey my thoughts good enough. But just in case. Previously we saw Hermes is when he promised to guide another traveller to the hotel, playing strictly and sharp on the MC's side, and even giving a speech of how much MC would delight him, if something bigger than partnership would happen between MC and Asterion, showing his affection to both MC and Asterion. The later he even compares with his child. Next we see him in the hotel during the opening night acting strange, avoiding MC, and even plotting something sinister against Asterion himself, according to Argos' words. Which is hardly consistent with the character. That's why I think it's some outside influence.