Loligran still have child quirks after 3004 years, one should at least act accordingly after a century but it's a running trope I guess...
Blemzapp acting up for stealing his thunder instead thanking for saving his ass was probably why they obviously kicked him out of the party, not he left on his own.
The "evil god" most likely was another isekaied person and people he influenced killed him through jealousy like we see how they leach on Yuusuke propped items, then spread lies which grew into religion.
This should help Yuusuke with dealing in dilemma to be with Sun, on the other hand, perhaps if isekaied person fulfills his role is then sent back to world of origin... that would suck...