I know this will be an unpopular opinion, but if DAR sent it back for handguards and it came back with scratches on the barrel and gas block, this sounds like it would be reasonable assumption that this happened during the removal of the gas block. I'm not saying I support the customer service complaint, or the response to it, but to say that it is unlikely that it was damaged during service seems naive. I can understand if the gas block wasn't installed properly and during shooting it started moving, it could cause the handguards to fall of, as well as rubbing paint that wouldn't be seen until after the gas block was put back properly. In retrospect, it would have been best to send it back to DPMS, but unfortunately once DTI took responsibility for repairing the rifle, they also accepted responsibility for how it was returned to the customer.
Personally I was nervous the first time I took a hammer/punch to my AR, since I didn't want to scratch the finish. Once it was scratched, I felt relief, and now I treat like a battle rifle, which is what it is.
If it's a safe queen, don't buy DPMS. Otherwise drop it in the dirt some and enjoy the durability, or buy some birchwood casey and touch it up.