Here's a thought, could you plug the hole in the stuck ball with a piece of all thread by screwing it in the hole in the ball then push a small powder charge into the touch hole and shoot the ball and all thread out. It's kind of risky but might work.
Or use a piece of all thread screwed into the hole in the ball and set up a slide hammer on the all thread that's sticking out of the barrel. Sort of the same idea as shooting it out but probably safer.
Sounds like you are making progress slowly but you are getting it out a piece at a time.