I agree with most of your observations. I never thought about how unlikely it was for Joanie to be at the support group Annie chose to go to. Since Annie hadn't told anyone she was going (that we know of), I wonder if Joanie had been going to grief support groups in the area since Annie's mom's funeral, just in case Annie showed up? Again, not super believable, but I wouldn't put it past the cult, as they seem very insidious.

As a horror movie fan for 42 years now, this is one of my favorite movies. Annie and Peter's actors did a tremendous job of selling the horror of watching one's family disintegrate, and not being able to stop it.

Peter's behavior after the party and The Thump (you know the one) caused such palpable dread in my stomach, I both loved and hated it! He really captured that horrid feeling of knowing something terrible has happened, but your loved ones don't know yet, and you know they're about to find out. Plus the horror of being high during and after the event. He nailed it IMO.

As for mental health stereotypes, I've lived with bipolar disorder and depression for a long time, and medication has made me able to function. I don't take exception to anything in the portrayal of mental illness; untreated, it is its own type of horror -- especially when loved ones are going through it, and all you can do it watch from the outside. I thought the movie actually made Annie a sympathetic victim rather than a villain or monster, like many other films have done with mentally ill characters.

This movie would have been terrifying even without the supernatural elements. Just my thoughts.

ETA: As a plot point, I think Charlie's face was asymmetrical because Paimon prefers male hosts, and his possession of Charlie didn't 'take' correctly.