I finally got ahold of an endoscope device that hooks up to a laptop and was able to look down into the spark plug socket. There I found no obstructions and while the threads looked relatively clean the focal length of the camera made inspection sort of difficult. The photo to the right is the top of the piston.
This really only leaves me with the conclusion that the threads are crossed and need to be chased. This is a tricky situation. Chasing the threads will probably result in some metal shavings which, if left in the cylinder, will potentially find their way into parts of the engine that definitely shouldn’t have tiny bits of sharp metal floating around inside them. I think if I grease the chasing tool up really well most of the shavings will remain stuck to it on the way out. The remaining ones that happen to fall into the cylinder can be cleaned out. I’ve heard that compressed air works fairly well at this, and the endoscope also has a magnet attachment that I can use to pick up the remaining bits. I’m nervous about all this, but also confident that I can do it. Wish me luck.