The reason that 'bum' might fit better is it goes along with English style of slang and language used in Hyde's 'cozy cannibalism' genre and setting. It stands out to have American slang terms used, just like it would in other very English things, like hiphop in Harry Potter. Its not a big thing, bit something to think about.

The oiling was mentioned by the main character in the first part, so forgetting it does seem a little inconsistent. If it was edited in with a couple of sentences, there could be a brief mention of both the spit and bottom being oiled. If Mollie is going to the effort of being humane with the stabilizer cuff, then it doesn't make sense to use a dry spit straight after that.

It seems that the story jumps a bit from the intro straight to the Caterers and then the Party. Having a small section between home and the waiting room where Timothy is introdcued a bit more, and even talks about what is happening to his friends at school could provide more of a build up.

The skewering and belly piercing take up more time than 'the main event', which might be something to think about in future.

Having even a little conversation with the two previous occupants of Prep Room 3 in part 3 or having the questions by the brother and sister in the waiting room actually asked instead of described could add to world building.