And I also would say, if anyone immersed the coin ane let it sit inside the solution, it would smear the ink as well on the private key.

That's what I thought would happen, but it didn't. The ink was definitely degraded but legible. This was a 1 BTC brass Casascius.

And if anyone REALLY wanted to get to a Cas private key, why not make the same Hologram and find a blank Cas and use the new hologram?

I think this is the most obvious and likely exploit.