Is it possible to verify and relay an encryption key with a trusted but transparent third party?
I'm not sure what you mean by transparent.
With a trusted third party what you ask is trivial. But with etherium I believe you can't, the key would become public there is nothing secret on the blockchain.

If we wanted our own distributed third party this can be done. You can perform secret sharing and send part of the key to each member of the distributed escrow. They can then perform shared computing to verify the secret without revealing the secret.