I do not believe for 1 second that 7 out of 8 injectors failed simultaneously (or at least all within a short period of time). I find it interesting that the OP could run well for periods of time, then lose power. Do I understand correctly that clearing the codes solved the problem, even if it was just temporary? Those trucks have a tricky emissions system with multiple sensors. If the Duramax gets too much carbon buildup, then the computer will automatically reduce the power until it clears. You clear it by driving at highway speeds, but not under load.
I hate to hear this for the OP...I really do. But I think the dealership was either incompetent, deceptive, or just a downright bunch of liars. If they would have told me that 7 injectors were bad, I would have packed up and got out of there fast. 2nd opinions are cheap, dealerships and good mechanics are everywhere. Our close family friends had power failure in their Expedition. They took it to their very reputable mechanic who is well respected and they've always gone there. Mechanic couldn't figure it out and told them that the engine was going to have be pulled and partially disassembled. Unhappy about such a prognosis, they took it to a shadetree mechanic they knew who quickly determined that the only problem was a $60 failed coil.