I have thought of another option. I will hold it in readiness especially in case someone enlists a crew member who may be inclined to help them take my seat...

Airlines offering pre-assigned seats must allow people to select a seat that accommodates their particular disability or medical need. (this is why Southwest can't limit preboarding or make people pick only certain seats, the flip side of this regulation). All under 14 CFR Part 382. They also can't ask what is your specific need or disability. Therefore, if you state that per part 382 for federally covered reasons you have been given this seat assignment, they can't legally move you unless providing an identical seat (which let's face it, providing same recline, same side of aircraft window or aisle, non-bulkhead or bulkhead, etc., leaves very very limited options and will require relocating other people - which the airline also isn't allowed to do once on board to accommodate you under same regs). Makes it a problem that a crew member during boarding is certainly not going to have the bandwidth to deal with.