[V]

how about this...
steel plate, bend to a sharp 90 degree, trim/bend to fit up in that corner
then cut out a section on the side to recess the nut into. just enough that the upper hole and the hole in plate and hole in nut all line up.
then have the nut welded in place onto the steel plate
the pics are the best i can do i think to explain the idea, with the red lines being the final weld lines.
with the nut itself still being flush up against the metal tray/seat back brace, you should have enough clearance since you said you cant thread the nut as it is now, but did you try holding the nut then threading the bolt into it. That way the hexagon bolt wouldn't need to turn, which reduces side clearance.
...its an idea...lol