M-m-m-m-m-most situations of one-way car rentals entail two divisions of what are truly the same company. When they're across an international boundary, sometimes that isn't the case.

Also, one runs into additional trouble given some regulations which restrict renting a car in another country and bringing it into one's own country. (this more troublesome on the southern US border than on the northern, but the rules are similar in many cases)

In your situation, Spokane and Calgary are just 400 miles apart, so IN the event you can find anybody who will even let you do it, you could do a one-way rental for a day, drive the whole thing, and then get another vehicle for the rest of the trip - that being a more standard and less expensive rental. (this only works if the high fees are added to the daily rental rate and not in the form of a one-way drop-off fee)

Given you're talking about an international flight, how do the costs compare to, say, flying into Missoula or Great Falls instead of Calgary?

Of the spots you mention Coeur D'Alene seems least enticing, and without it there would be no major need to go to Spokane. Maybe beginning and ending your trip in Montana, with either a same-city pick-up/return, OR a Great Falls-Missoula one-way, would fit what you need?

Missoula, MT to Jasper, AB is 560 miles while Spokane to Jasper is 502 miles. IF renting in your own country, you would likely find it much easier to be allowed to cross the border with a rental car.

Hope this stirs your thoughts toward a solution.