The ones who ask the seemingly stupid questions—the questions that no one else dares to ask—can sometimes make the critical difference, so let's do this ;-)

Is there any way a "mined" private key (vanity .onion) can be shared with another person (for that person to use it to host some service), WITHOUT requiring full trust in the person who originally found the key?

Without further precautions, I assume the person receiving the key has no guarantees that the person who found the key didn't keep a copy to then take over the service (or to at least surveil it in some form).

I don't see how this could be made trustless, but maybe my superficial knowledge of ed25519 is not sufficient to see the relevant nuances.