Norman's grandmother had the power.It makes sense honestly. She is Mr. Prenderghast's mother and she passed it down to him, also giving the gene to either Norman's mother or father. That's why she promised to stay behind to look after him.
Aggie hasn't moved on yet.Her unfinished business was getting revenge on those who wronged her, but where does it imply that giving up the task means it's done ? Aggie simply went to sleep on Norman's shoulder, but until she does get revenge on the judge, she's probably going to be stuck here for a while, even if she doesn't want revenge.
Agatha's mother was the ghost that she was talking to. Agatha's mother was dead before Aggie was found to be a 'witch.' And it was talking to her mother's ghost that got her caught. It makes the trial and execution all the more tragic, because Aggie did have someone who loved her - just not someone who was alive anymore.
The Animation Age Ghetto is the only thing that let the PG rating fly.Blasting zombies to pieces with guns, repeated swear words, child executions, and not throwing crap past the radar but throwing it straight at the radio tower, how did this movie get PG rating? Simple, it's animated.