Other than my desire for Snape to kill Dumbledore, I don't see any reason why HJPEV should talk Draco into letting him out of the promise.

It is more important to the themes of the work for HJPEV to follow through on a promise so dramatically given, than to shirk it. Likewise, it would be important for Dumbledore to face the consequences as administered by HJPEV.

You have an interesting point about the promise. It is awfully detailed for something that would just be set aside. Still, it could have been so detailed just to allow a semi-light character like HJPEV to bond with a semi-dark character like Draco. Or maybe to allow the author to demonstrate the practice of thinking things through, through HJPEV. Or, as the Pedant One points out, something else entirely.