I'm starting to side with the dealer on this one. I checked tire rack and they do not sell an all season or winter tire for the 19" M Sport wheels. That's really a bummer.
Do you have a discount tire nearby? If it was my time and money, I'd swing by and see if they can help at all.
My approach might be to see if a 265/50/19 all around would work. One size wider on the fronts is nothing - they'll fit. Running a 265 on the rear might be an issue - that's two sizes narrower than stock. The tire would easily mount to the wheel and ride just fine, but I would definitely be concerned with the looks of it.
One way or another if you try to run a square setup on those wheels you are going to have a visual contrast - the fronts are going to have squared shoulders, and the rear tire's shoulders are going to stretch inward. It might end up looking pretty bad.