Why not just drive from SEA straight to Whistler? That would get you to Whistler about the time the earlier flight arrives in YVR, never mind the later or much later flights. And with multiple people in a rental car compared to buying multiple plane tickets, it may be cheaper, though it depends a good bit on how you plan to get back. (A one way car rental from the US to Canada would likely be very expensive if it’s possible at all.)
I wouldn’t worry much at all about skipping SEA-PDX as the last flight on the ticket as a one-off. I would worry more about having an impossible booking on AS (flying SEA-PDX and SEA-YVR at the same time), especially if your frequent flyer numbers are in both reservations. Not a factor if you buy the AC flight.