I supect that you have rounded the surfaces that have to remain flat. Rather than use a file or especially sand paper to dress the two parts, I use a scraper I made from a file. It is about 3/8" wide and I ground a graver type end on it with a slight heel. I installed a good handle so I can really press and remove the high spots that show transfer colour. What is likely holding your parts apart and not letting them seat is the end of the hook striking the bottom of the female recess up at the top next to the tang, and also the mating edges of the hook where it extends out of the breech, and the corresponding edges of the tang's recess. The graver and scraper work well in those tight areas so you can stay well away from the flat surfaces and especially the corners. It is my opinion that power tools can get you into irreversible trouble almost immediately.
Rather than wiggle the tang around on the hook which will give you completely false 'high' spots, place the tang over the hook and give the tang a tap with a medium sized hammer. Then remove the tang carefully lifting it straight up, and check for contact that prevents the two pieces from seating.
You may have passed the point of no return and need a new breech plug and tang. Removing the plug from the barrel will in no way damage the barrel's threads unless you have either cross threaded it or overtightened it.