Making it part of the main story makes more sense to me, since I don't see how contracts would affect the MC's constitutional ability to call whoever he wants into the hotel, in this case a therapist specializing in dealing with trauma. I'm going to go against the grain here and suggest a human therapist makes far more sense than a mythical. I don't see how a mythical would be able to do well in that profession nor have the necessary experience given the major drawbacks of charms and how mythicals are treated in society. I think it's time for a good human character other than the MC (Greta is not a human) in the cast, and as RockJock pointed out it would be nice to have a non-magical being that can help in ways the magical beings can't.
As for how long the content should be, beyond the therapist's introduction it would make sense for it to be something ongoing in the background to be brought up in conversation between the MC and Asterion, which would accurately reflect how therapy is a gradual process.