Bolts at the factory are run up with (used to be air powered) electric torque controlled guns. Each gun has a controller that monitors the torque and shuts the tool off at target torque. Also can tell if it was cross threaded (not enough degrees). Fastening strategies are specific to the fastener, some are straight torque, some are torque and angle. Its a science, and the controller cal's are set by corporate. The controllers also record the values and report them to a host computer for monitoring the site wide performance of the entire process. Failed fastening operations set a light, and the QT has to come by to inspect, repair with a calibrated hand tool.
There's no guess work, or slack employee input. They try to eliminate operator error for every build operation. Many fasteners are MVSS spec, and have to be recorded by law (well all are recorded now). Part of GM's quality initiatives.
Possibility of poor materials that stretch over time, and this would get tagged by the warranty process. That would generate an internal investigation, and possible recall or TSB. Be assured when the vehicle left the factory, the fasteners were at design spec.
I agree the dealerships are the weak link, as the tech's experience and perseverance will determine whether the customer is satisfied. This is a tough problem though - yet they do have vibration monitoring tools available to them. Possible it needs to be escalated to district rep before they really dig in. Onus is on the dealer though to resolve. You install aftermarket parts and complain, that complicates the process, and is an easy out for the dealer - but you knew that going in. Dealers push back because they only get so much $ back from GM for a certain repair code. You need to request escalation, if they don't get it first shot, or 2nd at most. Its a game. I hate it, but if you know the game, at least you can hold them to their playbook.