Why would you try to clean it while it’s loaded? I think the author of this article knows little about how to clean a gun. Something is amiss here. This sad story makes little sense. Think of all of the mathetmatical possibilities that would have to coincide for this tragedy to occur.
How else does one unload a muzzle loader?
Still, not to be certain of your backstop is negligent.
The easiest way to remove the load is to fire it.
Otherwise you have to use a screw and try and screw it into to bullet and pull it out of the muzzle, or you can try and put an air hole on the nipple and try and blow it out.