You could slot the plate on both sides of the missing nut, then use a chisel and go in it at a 45° angle and start walking it around. Once it starts to move you should be OK you’ll need to buy a new piece because that bolt head is part of the whole plate, apparently it was tighten down too much. There’s an O-ring underneath a jthat plate. It just needs to be snug you don’t need the bull it