Revolving the plug is the best way to check the treading for deterioration and the bore for use.
I have been a BP gunsmith Professionally Pulling a plug for me on this gun is no big deal. If it was a CVA I would not attempt to pull it.
I have re-barreled at least 15 H&A actions in the last 30 years. They are not new to me. I have 20-30 50 caliber Knight rifle barrel screw-ups they at GM (who made all of them) cut the actions part off. Bores are sweet, the barrel outsides need refinish of course do to dings, bangs, and light rust.
I do however get very nervous over any long loaded BP barrel as until they are unloaded completely they are very dangerous.
I have many new breech plugs for the barrels I made in my shop 20 years ago. Sorry none are for sale.