Just in case I didn't make my meaning clear before. I don't think there ever was any reaction of any kind with the silver. I'm convinced that what you've got there is nothing more than a coating of "something" that is sitting on top of the silver. An apt comparison would be paint on metal. The paint causes no reaction in the metal - it merely coats it. And no I'm not suggesting it actually is paint, if it was paint it would have been dissolved by now. But at some point, whatever that stuff is, it was in liquid form, somehow got onto the coin before it was packaged and dried there. I suspect that if you could remove it, you'd find the metal underneath little changed.
If long soaks in water, acetone, xylene, and ammonia haven't substantially affected the spot, I'm running out of ideas. It looked like something was happening with the ammonia -- did you see any sign of white stuff diffusing into the liquid, even if most remained on the coin? If none of those work, I'd start to think about scraping. Actually, I'd try first to get the best microscopic view I could, just to see if I can tell whether it's sitting on the surface, dug into the surface, or part of the surface.
Yes, when wiping with Q-tip the ammonia gets cloudy. I will continue this process until there no longer seems to be a gain. Had minor surgery today so not sure if I will do much today besides rest on the couch and likely take a few naps. Nothing significant and hopefully tomorrow will be up and about.