Stupid me did not check that my car didn’t have the split folding seats (a.k.a. BMW Through Load System) before buying it...

Want to take out the rear seat back so I can load something long (a.k.a. Ikea), but I can’t seem to break loose the T50 screws at the base.

Any tips? I only have a screwdriver kit with the bit and a 18V cordless driver/drill.