Pull the charge with a wood screw welded to the end of a long rod with a slide hammer on it.
If that doesn't work, put a drill on the end of a pilot'd rod and drill the bullet and load out.
Once it's unloaded, chuck it up in a lathe and bore the breech plug out while cutting in reverse. At some point the plug (or the remaining threads) will break loose and you'll be left with threads you can clean up with a tap.
As for the nipple, just drill it out by using gradually increasing drills until you're at the minor diameter of the threads. CVA nipples are a 6x1mm thread, unless its been retapped at some point, so your minor diameter will be a 5mm drill, which is about halfway between 3/16" and 13/64". If you can, use a left handed 3/16" drill bit, which should also get the remaining threads to back out.
After that, mill and thread a new breach plug, install a new nipple and you're good to go.
Edit: Alternately, send it to me, I've been looking for a donor barrel to make a 8" volley gun from