Actually, it looks like if I fully extend the seat belt I can wedge it under the folded over rear seats. That opens things up a little.

We'll see tomorrow. Attempting to transport the seat from a three piece suite.

Can you bungee cord it to the side somehow? It won’t completely get it out of the way but maybe enough to give you room.

I’d be reluctant to unbolt anything that is obscured by trim (cue a future thread on recovering lost spacers in the roof lining). However is there anything in the manual about removal for large loads?

All the belt mountings I have seen (not that many, admittedly) are easily unbolted from threads built into the bodywork. If you can see any washers etc, behind the head of the bolt itself, there should be little worry about removing it from the roof. At least any loose bits won't fall upwards. Such fixings have to be removable in case of any necessary replacements.