The killer is obviously Kent. He’s the only one small enough to escape through the vent. He simply used forced perspective to seem larger. Kindaichi couldn’t tell because the hole is on the floor and he could only see with one eye.
Kent even alluded previously to the fact that he would cast...
I think they're trying to introduce peaceful monsters. Fish costume guy is obviously doing normal work, and the white ooze one didn't even want to hurt anyone until he was told to.