Wow, I just checked around on other websites, and apparently the seats getting stuck in the down position is a real problem happening quite often.
("The floor-level nut is spinning free") If the bolt is turning but not coming out, that means either that the nut's thread (either as part of a nut or metal plate) has detached itself from the car's body, or that the bolt's thread has been stripped. As far as I know there's no nut, but just a threaded metal plate that's welded onto the body. If the (nut or the) plate has detached (which I doubt), then if you do manage to get the bolt out, the nut/plate will just drop away - requiring access to that area anyway. It sounds like the bolt's thread is stripped, which is strange because those are very strong threads.
What I did when removing bolts with stripped threads (they were rubbish bolts though) was to get a crowbar with a "V" shaped end that grabbed behind the head of the nut, then I pulled the nut away from the object/nut/plate while turning it to loosen. A flat-bladed screwdriver might work too. If you have no room, try anything including pulling on the seatbelt anchoring, while trying to loosen the bolt. It has always worked for me, with enough quietly muttered curse words. Worth a shot, right?
