I'm not really up at all on the tech lingo for various security equipment. MMW?
If I used the Vancouver airport I would be flying internationally, not to the US.
What I'm thinking is:
Take train to Vancouver
Fly Vancouver to Franfurt
and then connect to my final destination
With the reverse itinerary on the return flight. Fly into Vancouver from Europe and train to Seattle.
Also Vancouver seems to have a better selection of international flights than SEATAC.