An off center chamber will definitely cause pressure issues as the bullet changes directions trying to find the bore. I've seen it many times over the years. I'm guessing they didn't proof fire the gun, or they should have seen the pressure signs.
This is a cheaper alternative to the tension lever:
https://t-nuts.com/index.php?cPath=73&osCsid=10af9d230c330ef37072f92623fc4b80
I'm working on setting up the machining process for some bipod feet. They should be for sale in a few months. 😎
A lot of shops and end users don't know that SAAMI has published exact specs for muzzle threads (as I posted above). Following these guidelines would eliminate a lot of ambiguity. I never said all machinists were competent which is why I qualified the statement with that adjective.
While I can understand not letting someone use my barrel as a guinea pig to learn barrel threading, it's really not a difficult process. Any competent machinist should be able to do it.