The company I work for makes those door panels, and I can assure you any level of rattle is absolutely not acceptable. If that rattle were detected at the assembly plant during any of the extensive squeak and rattle testing they do on a regular basis our engineers would be hauled in there and told to get it fixed. GM does not consider any squeaks or rattles to be acceptable on any vehicle.
If the door trim panel is rattling, it is not installed properly. They are designed to have a 2 to 3 mm crash fit to the sheet metal, which means they edges of the panel spring a little when installed and create a preload on the door clips to keep the panel tight to the sheet metal. Lock rod rattle is a very common problem which is usually solved by a piece of felt tape applied at the rattle point. If these lock rods are susceptible to rattle I am sure there is supposed to be a counter measure installed in the panel (like felt tape).
Based on having gone in to GM assembly plants to fix these types of issues, I would start my investigation by looking at two things. If the lock rod is rattling, the counter measure may be missing, or installed out or position. The remedy would be to apply a piece of PSA (pressure sensitive adhesive) backed felt tape or foam to isolate the lock rod from the contact point. If the door panel is not seated properly, I would suspect it had been removed and re-installed at some point and the clips were not replaced. The plastic clips that secure the door trim panel are supposed to be single use and are to be replaced if the panel is removed and replaced. The remedy would be to remove the door trim panel, install new clips, and re-install the panel.
Continue to elevate your issue until it is resolved. Door panel rattle in a new GM vehicle is absolutely not an acceptable condition, and the people telling you your expectations are too high need to be smacked. The people telling you they don't know how to fix this should to be fired.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)