There is actually a GMC dealer in Idaho Falls that could also have looked at your truck. I am surprised that you did not know that since they are right next to each other. You can throw a rock from one to the other. There is another Chevy dealer 15 miles north of Idaho Falls in Rigby. I have bought several pickups from the Chevy dealer in Idaho Falls. I have had decent experiences with them but I cant say that they not capable of making a mistake. I agree that $7k sounds too high. I certainly would have gotten another opinion. There are GM dealers all over this valley.
I am trying to make sense of your trip so I have a couple questions. You stopped in Missoula to get it checked out, than it needed to be reset 8 times from Boise to Arco (which is Idaho not Montana) but how did you get to Boise in the first place? That's a good haul, its 5 plus hours from Idaho Falls to Missoula going over the pass near Salmon and even longer going down I-15. It is another 4 hours from Idaho Falls to Boise. You may have saved some miles if you went through Challis from Missoula to get to Boise. How did the truck run from Missoula to Boise and what route did you take to get there?
It is 260 miles or so from Boise to Idaho Falls via the desert route you took. Not sure how you logged that many miles with 7 bad injectors. The entire story is hard for me to wrap my head around.