One way on August 3rd on WS is 144.87 all in. On AC it is 144.87 on Tango with all options included.
About SEA-YMM or YVR-YMM, AC from SEA is cheaper (196.86 on Tango Plus) than WS from YVR (264.65) and I would also need to get to YVR, not only SEA.
I called them and unfortunately the credit can only be applied to NW/KL itineraries only. They also could not find the itinerary I wanted - only YMM-YYZ and then KLM after. But the price was about 800 more than what I find on ITA.
I guess the answer to this is to book it separate or with a travel agent.
Thanks to all of you for info.
Important info I left out is that I leave on a Friday which blows up fares from YMM on WestJet or something like that.