O.
M.
G.
I’m going to go crawl under a rock.
I had forgotten those settings existed. I imagine when I disabled them I was thinking only of notifications like “an admin has added a tag or changed the category of your topic” – not actions like Watching or WFP notifications.
I think there may still be some inconsistencies, as Watching a category seemed to override disable category edit modifications, while WFP did not. As penance, I will test this more thoroughly and follow up with a couple of other people I pinged on the subject.
(Also, @sam did kind of back up my conclusion 
)
But thank you @JammyDodger for rescuing me from more hours of puttering with rails scripts. My next step was to sell my couch & pay @pfaffman to help.