Unless you have left out some important info, OP, this seems fairly simple.

YMM-YEG is a Westjet market, and they have much lower OW fares than AC does. Same for YVR-YMM.

Why not just buy a couple of OW tickets on Westjet, and then use your voucher for a NW ticket to/from OSL?

You just have to compare whether one ticket is better than, in this case, 3 of them. Add it up, look at the schedule options available, and choose what works best for YOU! As always, sometimes the lowest fare is not always the best choice!!! Like if it means spending 8 hours in some awful airport twiddling your thumbs.