Well, it is certainly counter-intuitive that a thimble full of water could double the pressure in a billion gallon tank, but I guess in practice, it would require to be topped up again and again, as the increased pressure caused slight expansion of the tank.
Yes you could. However, unless the tank was made of an absolutely rigid material, it would deform and the thimble of water will enter the new volume which would drop the pressure.
Yes, I alluded to that, but it is incredibly counter-intuitive that a cup full of water could double the pressure in a million gallon tank. Now, if I had the million gallons totally sealed, no air, and injected the cup of water through a non-return valve, the pressure difference in the tank would have to be minimal, as the tank yielded ever so slightly, would it not ?