How do I make sure BookyAI only ever writes with the engine and model I picked?

Open Settings and find "If my engine fails". Choose "Stay on my engine". From version 1.9.21 this means BookyAI never switches, not to another engine and not to another model of the same engine: your engine and model are tried again after a pause, a chapter that still fails is marked as failed, the book moves on, and the failed chapters get one more pass at the end. Anything left can be regenerated from the book's page. The other choice, "Continue with another engine", lets another model write the chapter so the book always finishes; you are then told in the progress line and the chapter's label turns amber.