Pour Tap cutting oil into the hole after removing the drum. Then take it easy as the breechplug is turned Slowly, BACK AND FORTH, to knock off burrs from that cross threading damage. The Tap cutting oil will help prevent damage to the threads in the barrel for the plug.
The near side threads, as the plug is turned out, will see the burrs smoothed out towards the hole, where you can use a fine rat-tail file to knock them off, and dump them out. When the plug is turned just a bit, the following burrs on the threads are exposed in the hole, and a fine jeweler's file can be used to remove the burrs, and dress the thread so that they enter the thread on the other side of the hole when the plug is turned further. Just don't force anything, save the initial breaking loose of the plug from the barrel threads(which can't be helped.)
Once, the plug is turned 90 degrees, so that the hole for the drum is now completely covered by the threads in the barrel, turning the barrel can proceed normally. If there is a " Hitch", stop, back up the plug, and take a look to see what is blocking the rotation.