Drag Me to Hell/WMG

Christine was Dead All Along

She died during the garage attack. The entire rest of the movie was a Dying Dream as Mrs. Ganush dragged Christine's soul to Hell with her.

The Lamia is a Deadite.

When people get possessed by the Lamia, their facial features distort, they threaten to eat souls, and they can levitate. It doesn't want to drag Christine to Hell, just to the Worlds Beyond.

The Lamia is Ai Enma.

The three days in the movie are just a very long banishing sequence. Alternatively, the Lamia is actually the spider, fulfilling a request while Ai is on a job somewhere else. This would also explain the kid's (rather sadistic) damnation since the spider is not only Ai's boss but also the Lord of Hell in Hell Girl and also kind of a prick.