Agree with lars46. The smallest AWD is the Suzuki, it’s smaller and lighter than an Impreza. But it won’t get 50 mpg. In fact nothing available in the US will give you 50 mpg on an ongoing basis in mountain driving. The closest is the Prius, and it does not have very good traction with those skinny tires.
Most of life is made up of compromises, and a light 2 WD vehicle with good snow tires is the best bet in your case. Practice sane driving and your mileage will go up as well.
Under no circumstances would any of us recommend bringing in a high mileage diesel from abroad, even if you could get it past the US EPA inspection (not likely). You will have no end of trouble getting it serviced.
We live near the mountains and spend every weekend there in the winter. Our best car is a Corolla with Michelin X-ICE winter tires. We have 5 Subarus in our ski club, but they don’t seem to have any significanr advantage over the rest of us.
I would highly recommned a $150 winter driving course as put on by the AAA and other organizations.