Agreed on the bolt not being torqued correctly, plausible cause of damage!
I agree the rear brakes , in a drum rear brake HHR not adjusted correctly causing brake judder.
I have found over the years some brake shake caused by a toasted rear lower control arm bushing, even after I swapped in the larger diameter SS non Brembo rotors and caliper brackets.

 

The caliper brackets are just fine, and don't flex. That is the first time I've ever heard that one, and I do Chassis warranty at an OEM!

It sounds like your bracket might have been loose.