Ok great, thank you!

That could have been a star drive or hex drive head that someone stripped out when they were tightening it. If you want the entire fastener completely out, the vise grip approach would also be my first approach, but that head may be too thin to get a decent grip. Drilling with a couple of bits starting at just slightly larger than the hole diameter should allow you to get the head to fall off, and the screw shaft that remains in the wall may then be approximately flush with the surface.

I think it's a "Spax" brand wafer head screw with a star bit head. It would be good to find the right bit for it as these screws are heat treated and tough on drill bits.

IDK - really looks like a pop-rivet to me!

Not sure what it is but another approach would be to grind the head off, pop the L bracket off and use vice grips to on the remaining nub to back the screw out, if it is a screw.

Damn dude get yourself a new pair of pliers! LOL!!