Beyond Selidor

You are so right about episode 4! I hadn't thought of it in that way (and honestly I hadn't picked up on everything you mentioned, probably because my family's dynamic is very similar except...moreso).

seem to come from people who want everything spelt out in blazing letters. It's as if they wanted a courtroom scene in which Jamie suddenly admitted his guilt and motives like a comic book villain speech, or for some other character (the psychologist, or perhaps one of the two police officers) to almost turn to the camera and tell viewers the motives in unambiguous terms.

Absolutely. They've been trained by other shows to want that, too--that often happens at the end of, like, a Law and Order episode.

Your understanding of the timeline is the one that makes sense, but I think it is implied in the show that he didn't know what the 80/20 thing was until Katie used it, which seems highly improbable to me if he'd already been in the incel stuff.