I put some heat on that black build-up ring with a woodburning iron and sure enough, it began erasing away. It's actually pretty thick. I'll finish it tomorrow with a different heat tip, then smooth it out.
I've been looking at the extractor and spring, and I'll probably replace the spring.
I just bought a replacement gas piston. The piston in the gun now is still within the correct specs, so I'll just keep the new one aside for now.
There's stories floating around about the gun being unsafe - people supposedly have shot themselves while holstering/drawing. If true, they probably did exactly what you described - their finger was on the trigger when they cocked the gun.