I can feel it in my bonesIt's Mima 🌙 Trust me!
I dunno if this'll help, but the idea of "youkai are especially weak to vengeful spirit possession" has been in the lore since like 2012, from Symposium of Post-Mysticism. So it's at least consistent world building, not just an asspull for the sake of this manga. Although if you don't like the general idea even from 2012, that's valid.I'm kinda miffed that all these powerful characters are so easily possessed by vengeful spirits. You would think that centuries of esoteric training, divine power, etc, would at least give them some resistance to such attacks.
The Copium part got me. HahahahaINJECT MIMA COPIUM DIRECTLY INTO MY VEINS