Well, once I bought one of these:
And one of these:
And one of these:
I was able to find this:
Sorry for the grainy quality of the photo, I had to take a picture of the screen with my phone.
After I soaked down the spark plug well with brake fluid, I was able to suck out all the oil/brake fluid mix with a Mity Vac, at which point I could finally see something jammed between the facet on the spark plug and the wall of the spark plug well. I couldn't dislodge it with a pick or with compressed air from the compresser, but I was able to get a grip on it with the alligator hemostat and work it out of its position.
And here it is....
It appears to be a very sharp piece of gravel that wedged itself into that space like a piece of a jigsaw puzzle. Without the borescope and the alligator hemostat, I'd have never been able to see it clearly, or been able to remove it; even the compressed air wasn't getting it loose. At least I have the tools now to be able to get a good look down into the spark plug wells before I remove the spark plugs.
And tomorrow I can go back to square one. I'll pull all the plugs again so I can look inside the cylinders and make sure that nothing manged to fall into one of them when the shop had the plugs out and the valve cover off the last time. I'll finally be able to pull plug #3 and see what shape it's in. A valve clearance check is waiting on a new valve cover gasket.