Any thoughts on the twist/ending? **SPOILERS**

I believe she was possessed by the native American mother of the little girls. She helped her family's spirits escape the mine. The rat and Brad may have been the sacrifice necessary. I dunno. AS to whether she is still possessed? I dunno.

I think you're on the right track, though I added another layer to it. The crux of the movie is when Laurie is sitting in that chair with the disembodied laughter. We go back to that scene, what, three times? At that point, I think the movie switches from having a third-person-omniscient point of view to switching to a third-person-singular--from there on out, we are seeing everything filtered through Laurie's state of mind. She could certainly be possessed, but a number of small snippets of dialogue throughout the movie had me thinking her mind had simply snapped. She THINKS she's possessed. ("Every time things get difficult, you just run away." Contrasted with her telling Ethan "I'm done running" after she's possessed/thinks she's possessed.) That way, the final sequence still works--she THINKS the ghosts are looking in at her (and she would've dropped the mask there herself, probably when she was on her way home after that night).

The look of despair on her face as she's editing the footage? Now everything's done, Brad has been buried, the "ghosts" are released, and the fallout from the event that her mind had seized on is finished... but she's still insane, and has no way back to sanity.

Either interpretation works, IMO.