Sorry to hear about your bad luck. I think the block heaters on this car don't have the best reliability. I had my original one replaced at 4 months and the second one is getting worse and worse at heating up the car. Will make sure to have it replaced again before my warranty expires.
I planned on keeping this car 7-8 years but with all the problems I've had, I can't wait to get rid of it. Block heater, transmission, MFT, squeaks & rattles, paint chips from mis-aligned body panels. I've never had a new car that required this much attention.
As for the oil, there isn't too much of a difference between 0W-20 and 5W-20 so if the dealer is being a prick about it I wouldn't let it bother you.