I just wanted to eliminate all such possible concerns so that no one could say "there will always be something you haven't thought of."

I can still say that. Layering sandboxes doesn't mean an AI can't find a flaw in each layer.

When you're up against an opponent that might understand its own systems on a much deeper level than you, adding barriers that look confusing to you is not the appropriate response.

A proven JVM would be a good a method of hardware isolation. Cryptography gives a false sense of security.

I have far more confidence in a system that simply denies access based on a bit flag than that a cryptosystem performs as advertised. If you can't do the former correctly, then I definitely don't trust you with the latter.