Well as the OP has pointed out the firearm has rust which is keeping the remaining part of the old lead ball stuck , speculation on what would have worked as none of us have the barrel or skin in the game , breech removal was never tried as he was worried about damaging it .
It is still an option , the question always is at what amount of force , wall hanger or will he try his idea of loading and seeing if shooting will remove what remains .
So to the original poster keep us updated on further developments .