I might start to look for another dealer. At three k miles the ideal of you being surcharged because they cannot find the issue is crazy. They have to or replace the car under common law in Canada I believe. You might want to talk to a mercedes zone rep or somebody.
Next time it really cools down take the car in the night before and leave it so it is cold in the morning. Taking it in warm for immediate code reading might reveal nothing. I know it is difficult out there but you do have a problem of some sort. Something like the cold start enrichment system is lean for really cold tempertures is possible.
Do make sure it is not a fuel issue as if they use that line on you they can probably bill warranty on the actual cause and then bill you for fuel problems. Or it could actually be fuel problems. The only issue here is yes you are responsible for fuel quality. Be careful and cover your posterior. Absolutly eliminate it as a possibility before taking it in.
I also would get this problem resolved this winter and do not leave it for next. Even mercedes does have to fix a car with 3 k miles. If I lived near you I could help you. One really good dose of me seems to light a fire under people if required. People have to act really unreasonable to force the issue for to occur. One dose of it though and they want no more. Basically if you have a dealer that tries to slide by at 3k you do not have a dealer. These kinds of attitudes are what kills off future sales. They are just stupid to start with. Your necessity to post on your feelings indicates this. There will be no second mercedes from them if this does not smooth out pretty quick.