But I am confident these physics issues can be worked out to a sufficient level of trust.
Only need to confirm that the private key was not communicated from the computer to any one.
I find this kinda weak against your general absolutism. "So Simple Yet So Complex".
After all, what stops all 3 letter agencies, who can own blockchains and can do analysis and attacks etc, to stage the whole thing? Will i be allowed to check that computer?
I mean, i have near to zero understanding of cryptography, but your search for the perfect/ideal solution looks like making you ready to take a huge and dangerous bet.