I’ve used two techniques succesfully, one’s the hacksaw a slot screwdriver one (usually my first choice) that you said won’t work with the tools you’ve got. When that doesn’t work I drill out the bolt head from the hex side.
Choose a drill bit slightly larger than the hex hole and drill down into the head, CAREFULLY. The aim is to stop just before the bolt head stops and the screwthread starts, go too far and you will drill into the disk. Then with a hammer and screwdriver/drift hit the bolt on the side of the head to shear it off (if you hit it on the inside of the drilled hole there is less chance off slipping and marking the disk). Undo all the other screws, remove the disk then get a pair of pliers/molegrips on the stub of screw thread and unscrew it.