You've already stated that you can't justify the M5 in Alaska, so that option doesn't make a lot of sense. And you're happy with the Denali, so swapping it for an X5M doesn't make sense, since you don't know if you'll actually like it more than the Denali. So doing the X5M doesn't make sense either.
How about either keeping the status quo or swapping the M3 for something that is drivable in the winter? Not sure what area you are in or what the conditions are, but would an AWD sedan be workable in the winter? Audi S6?