Removed left rear armrest with considerable difficulty, using a flat scraper. Roughly trimmed with the multitool and refitted, and now it goes on and comes off pretty well, and is located in position by the flange and locating pins very positively. I see only minimal need for fixing screws, and them only along the bottom overlap. I am pleased.
Put in a couple of hardpoints forward, and plan another screw through the seatbelt hardpoint. [Timewarp: Did not allow for the vertical bulkhead at the forward edge of the rear seat, it interferes with one of the hardpoints, and demands another on the other side.]