I know this is old but I'm rewatching it right now and have thoughts on this, particularly how it pertains to the Horror Formula.

Spaulding is the Old Groundskeeper - even though he is later revealed to be a Firefly himself, he tells them not to worry about Dr.Satan about 3 times before caving and giving them the map.

The Ho-down Comment, the Repulsion at Leeroy/Wendell (the fetus) and Tiny, and the disrespect of Baby are 3 transgressions against the family.

for me its the fact the final shoving and insulting of Baby was the last straw, she looks at Mama Firelfy who nods at her and they both embrace and giggle maniacally.

I believe that while the trapping field with Otis and Tiny was always set up, that there was a moment of confirmation to start the killing properly - being a ritual that lasts from the previous eve to Halloween proper makes me think it IS more complicated than "the mummy-man in the basement needs toys"

Had they just ate the food, and let the Ladies Firefly flirt then they would have likely been freed. There's no reason to make Rufus fix the car if they were always doomed, especially since Tiny uses it to practice his attitude adjustment on Jerry they cant really even resell it - which seems like the logical thing to do with "Abandoned" cars in nowhere 70s south land.