Not to be bitchy about it (didn't have a good sleep tonight so maybe a bit), but I had already replied with that answer yesterday...
you're right! this probably went down in the process of the thread. in principle, you have already laid the right track yesterday!
You could also export the private key (same tab, but private key instead of sign) and sign it on electrum (for example).
the proofs of coinlocket$ look comprehensible. this probably does not have a positive effect on the whole thing.
regarding the sign of the message we have done what we could. the rest is out of my hands.