The rearmost seat turned out to be worse than I thought; someone had cut the metal frame apart and made their own wooden thing, which was primitive and coming apart. I harvested fabric for future repairs to the front and middle seats and took ’em out.
The backrest of the rearmost seat hinged on two studs into the wall. I removed the trim panel and tried to get a grip on the nut I expected to find on the other side. I could feel it, but it wouldn’t budge. Felt around again and realised it’s welded onto the frame. Removing the seat was easily accomplished by turning the studs with a pliers (click photo for larger version)