I'm really confused.

She did have MPD. Mimi was the only one. it was said in the movie when Jane was talking to her, she got her to bring back the memory. Dorothy's mother wanted her to kiss the dead bodies, and I think Mary was her cousin in a line that was said by the mother. Dorothy didn't want to kiss the bodies and you see a little girl appear. That was Mimi and Jane told her that she didn't kiss the bodies, that Mimi did it for her. At the same time, because she was very vulnerable, Mary and her friends channeled Dorothy. The families of the dead kids were living there on the island as well as the people that hurt Mary. Because Dorothy is young, it was easiest for them to get her to help them as the families still grieve 10 years later. I don't think you missed anything. I think you might have separate the MPD from the kids that are dead. Since she was already unstable, the dead kids used her. Dorothy doesn't channel the dead, I believe the dead channel her as she has no idea what is happening and is scared half the time. At the end, she still has to have treatment for MPD as that takes a long time to treat, though it could be a shorter time since there was only Mimi. You also see Dorothy ask at the end if they are all gone. You even see that she doesn't want them around.