Lapping removes metal in 10,000 inch at a time, rather than even a thousandth of an inch. I don't think lapping is going to help if the damage is to the lands and as severe as claimed. It would take a lot of force to do that kind of damage. What were you thinking?
You may have to replace the barrel. Shoot the gun and see what happens. It might still shoot accurately, in spite of your concerns.
Once you move a stuck ball forward, just pour powder into the barrel by removing the nipple, and running some powder down through the flash channel. It doesn't take much - 5 grains -- to push the ball out the muzzle. Do, push the ball back down onto the powder, so you don't ring the barrel by leaving the ball forward to act as a bore obstruction. In guns with the Nock-style breech powder chambers, you want to use enough powder to fill that chamber. That may require as much as 20 grains of powder depending on make and caliber.
Remember that the gun is usually shooting PRBs, and the ball itself never touches even the lands. If you have a major scratch across a land, only the patch is going to touch it. Yeah, powder residue may build up in the groove of the scratch, but that is cleaned out with soap and water.