If High Priest man isn't the badguy, I just don't know... I think it is highly likely he's ruling over Ente Isla in the absence of the hero and the demon king.

I imagine the earthquakes are involved in either transporting people to Earth or siphoning out magic power.
Chi-chan's house is probably on a leyline or something, so I'm sure they'll all have to go visit her at some point.