I wouldn't worry about the cord really. Like myself, you're just gonna need to monitor the temp outside a little more if you are going to plug in your car. If the forecast calls for it to warm up overnight then i would leave it unplugged. If you do leave it plugged in and it's definitely warmer than -18 just unplug it and don't start your car for a half hour or so.
Regarding GM replacing your gauge cluster, i would definitely make a stink about that. I'm not sure exactly where you brought your car but i think they should have been able to figure this one out instead of replacing the whole damn cluster! I'd like to think they didn't try and pull a fast one on you but you never know i guess. If i were you i would go back and see if you can work something out.