I was going to post the same about the added duration, with the following additional comment: Depending on your travel dates, you can likely snag a fully refundable A fare NONSTOP on UA from IAD to SEA for about $1500. the nonstop might bring the travel time down such that the train is more palatable. It's not saving you any money from the AC fare, but it is refundable (I infer that the AC fare is not), which would allow you to continue to shop, and perhaps use the US MC certs after all (but don't forget there's a 14-day adv. restriction).

Was it just availability that caused your to require a positioning flight rather than book from WAS? If so, I'd want to have the flexibility to keep shopping b/c award availability between WAS and YVR might open, in which case you could get the award re-issued and not have to buy a positioning flight at all. [you might even look into that IAD/SEA routing with a ARNK into YVR for your connection] Edited to add: I missed the prior post about the stopover--b/c this is international, you have 24 hours to make your connection in YVR before it is considered a stopover, which could be plenty of time to connect even if you have a ground connection to SEA.