I see a problem with the shot glass scenario. I tried the nail up to 2-3" above the dime and was still able to pick it up, but beyond that it began to fail. The problem is at some point the dime is far enough away from the coil that the nail easily overpowers the faint signal of the coin. True this would be a "real world" scenario, but only one of many possible scenarios. No detector will be able to detect the coin once the coins signal becomes faint enough for the nail to take priority.
My test was done in my shop and with no adjustments to the gain(PAG/ACSens). I'm sure had I don't it outside and cranked it up I could have gotten a signal with the coin 3-4" below the nail. That is still a pretty respectable showing in my opinion. I also realize this air test does not depict what might happen in the ground. Here in southwest Kansas, our ground is pretty mellow so I'm sure I could get close to the same results in the ground. In fact I do get the same results as I have dug many coins out with nails in the same hole.
Now as far as the old TR's. I used many good TR units back in the day and while they were good at this type of situation, I don't think they would pass the 3-4" test either. The reason is, from my memory, it took a lot of skill, and luck, just to get 6" in depth. If you have a coin below the nail at 4" and you swing the coil above the nail at say 2", you're looking at needing to hit the coin at 6" from the coil. A good feat for the old TR's even without a nail on top.