If Takano was the one to die on the night of the festival, who went missing? Assuming Keichi is perfectly sane, it could only mean that the uncle was still barely alive when Keichi buried him, and he went missing due to the curse.
Another possibility is that Takano did see Keichi burying the corpse, and she went to dig it up later that night. The problem with this theory is that they found her body in the morning, so if she had the corpse, it should have been there with her.
In my opinion, the biggest question right now is who went missing?