I don't understand why this movie is nudging at your boundaries. Unless it is the theme? But, it is based in science gone haywire - not supernatural.
Anyhoo, While we have not seen it yet - actually plan on it for next weekend - I looked up a little about the movie, and I think in terms of cinematography it is really cool. Stop motion black and white with all the "actors" being hand crafted mechanical puppets. From Wikipedia - "The mechanics are small and delicate, and in some instances they had to have Swiss watchmakers create the tiny nuts and bolts. Around 200 separate puppets were used, with roughly 18 different versions of Victor. The puppets also have human hair, with 40–45 joints for the human characters and about 300 parts for Sparky"
I think that is just cool. So, maybe to ease you discomfort with the movie, focus on the mechanical aspects of the production when talking to your son about it. Eh, just a thought - we are production style freaks in my house.
Anyhoo, as for the Mom announcing changes in plans at the last minute, you are going to just have to say "No" the next time she does that when it is out of your comfort zone. Or, talk to her before the next visit and let her know that you are not comfortable and feel backed into a corner when she does that.
I am the spur of the moment mom when my son has friends over - "that" mom who would quick decide to go to a movie, the park, the pizza joint, the beach - But, if I knew that one of the boys Moms was not comfortable with the quick change in plans I restrained myself. Sometimes the boys would tell me that their Mom didn't want them to go anywhere other than what was planned, sometimes the Mom told me upfront.
So talk to the other Mom.
Hugs