No, it was supernatural. And the whole show was supernatural all the way through. It just didn't become clear until the end. We thought Linus seeing his mother's ghost all the way through was in his head; but then Aiden Gillen saw her too when he was being put in the police car. Seth was unbalanced; but was also right about the spirits and sacrifice and the woods. The whole thing was run through with Pagan and Occult symbolism: twins, stones, May day itself, curses and hexes. I understand why a lot of people didn't go for it, but I enjoyed it a lot, mainstream BBC trying to do something sightly different with one of their streamed five-part murder mysteries.
Ugh, it sounds as if many people did not pay attention. Caitlin and Hattie switch places. At the end Hattie is inhabiting the body and Caitlin is the spirit. This is why Caitlin tells Linus that she is going away and he will never see her again. Just before her murder, Hattie says that she will come back for the murderer. Also, Seth says as much at the church. So, the spirit Caitlin, not Hattie, is the one who switches the evidence as a gift to Linus. Linus is smiling because he knows it was Caitlin.