Christine was Dead All AlongShe 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.
Clayton didn't see Christine get dragged off to hellHe actually just saw her get run over by the train. Likewise, Shaun San Dena saw the Mexican boy die from his fall off the balcony. The vision of them being dragged to hell is only what Christine and the Mexican boy saw, not the bystanders.
The Ganush clan is also damnedUsing this curse may satisfy pride of place, but for anything other than avenging a murder or like crime, it places you and all you hold dear in the pocket of the demon you summon. This was also why giving the button away to Sylvia was pointless. By invoking this over a relative slight, she was already headed downstairs, and this caused the seeming mistake that shaped the ending.
Christine and the Mexican boy don't stay in Hell for long.Yes, they both get dragged to Hell. However, Hell is meant as the ultimate punishment for bad people. God eventually steps in and tells the Devil that, no, these souls aren't evil enough to belong here. Christine and the Mexican boy will never be able to forget their time spent in Hell, but they are taken to Heaven (or Purgatory) while Sylvia (and anybody who does the same thing) probably aren't going to be able to repent in time. Just let me have my Bittersweet Ending.