Hahaha, I like that the regular detective theorist girl throws a tantrum. It probably mirrors the author's own perception of people who "theorize" about horror stories, like... yeah, there's gonna be a supernatural twist you didn't see coming because who could have predicted that, idk, water is alive...
Speaking of that, I see the ending in very much the same way that I saw the end of the hot springs story. Not setting up a big long arc, but setting up a theme. "The horror is out there, the horror is real, the horror is uncontained." This story shares similar themes to Lucifer (2018)'s The Devil At Heart storyline, where the theme is that society is best contained when the devil exists. When people have a demon to pin their fears on. When terrible things happen, not because people are capable of doing terrible things, but because the devil is real and he is powerful and he moves our hands. To quote Judge Claude Frollo: "It's not my fault//if in God's plan//he made the Devil so much stronger than a maaaaaaan."
So yeah kids. Don't blame your failings on a demon possessing you.