Just my .02 cents here.
"...leaving from Missoula, MT and back home to Seward, AK by early February.
My rough plan is to head along US Hwy 12 through ID, onto I-84 to Portland, then up I-5 to Bellingham, and eventually ... "
Having made much of the same drive (out of Hamilton to the south of you) several times, UNLESS there's going to be a major winter driving issue just get on I-90 and head west to Seattle. You'll cut hours, and I do mean hours, off your total drive time. Even with 'adverse weather' the respective States will pile their resources into keeping I-90 open before they even take a shot at Hwy 12.
Basic reason for that is '90 is THE main transportation route going east/west through the northern tier of States.
Fred Evans - White City, Oregon