Since the dealer says the installer butchered the installation, I would think that's enough to allow you to prevail.

As far as the turning the engine backwards and bending valves, well that sounds like a load of bunk. Just imagine how many engines would be trashed over a simple backfire or misfire when trying to start the engine. The fact that it's bunk would also explain the lack of available information on the subject. There's not going to be any documentation if there really isn't an issue.