If your planning on replacing that barrel, now would be a good time. I would suggest you wait for Shawn to finish it up and then address the issues with the other shop manager. After DE is done you will have more info on what happened.
If your story is as it appears, they made at least three BIG mistakes. Turning it down despite manufacturer warning, not dialing in a few inches of the bore at the muzzle before threading, and installing it upside down. I just looked at the DE site and see the .750" brake has a top and bottom, any fool should notice that if they had it in hand. I would bet they don't have the rods to dial in the bore, nor a gunsmith even employed there.