How about doing it the proper engineering way – not the fitter way, with a clumsy hammer and chisel?
Drill out the core of the screw head. Job likely done. The screw is (or should be) scrap, anyway. If the threaded section is any problem, drilling carefully/accurately with left handed drills will solve that problem.
As regards the QCTP, they are good for what they are, but not as rigid as a 4 way tool post. The original lantern type were even better, but replaced, long ago, by more ‘convenient’ tool holding systems.
OK, I have one on my lathe, but I am quite aware of the limitations and extra care needed in some circumstances. Many, out there, think they are the holy grail of tool cutter holder. They are not in a great number of cases – particularly old, worn, flimsy lathes operated by those that don’t know any better.