Assuming you have no need for that frame, I would cut that tab or bend break it off, not too close to the screw. Then you should be able to get a pair of pliers on it and turn it. If it is jammed tight, that should cause the screw to break loose allowing it to unscrew using your previous methods.
If not you could either try cutting a new slot in it for a screwdriver or file the head off and forget about it. 3 screws will hold just fine in the new location.