Yeah definitely been messed with. Appreciate the feedback. Taking this slowI'm looking at that very buggered hammer screw versus a fairly virginal top jaw screw and wondering if the jaw screw was replaced with something with the wrong thread. If so, go gently, you might be able to restore the original threading.
As noted above, patience is the most important thing to apply to stuck screws. Repeatedly soak in penetrant and heat. Give it a couple of weeks!