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Depends on your threat model. You cannot, under any circumstance, prove (mathematically) that a peer is the only controller of a private key.

Again, I would love to know if I'm wrong.

The fact that no publicly disclosed threat actor has been identified says nothing.

Proving a negative that information has not been shared has been a challenge from the beginning of information.

Are you suggesting a solution for this situation?

In this case it's rather like trusting that a government issued private key has not been stored by the government for further use.