Southwest has a non-stop from SEA to LAS that leaves at 6:50 a.m. and arrives at 9:20a.m. They than have a non-stop that leaves at 9:25 and gets to BWI at 5:05p.m.

That flight in SEA is probably an originator, so very rarely delayed. I would bet you get to LAS early on a long haul flight like SEA-LAS. If you should miss it there is a 9:40a.m. departure via BNA. Gets you to BWI at 6:30 the latest. Book this over the Internet doing point to point and you got yourself 4 flight credits one way.

You may also want to check HP or CO for a connecting flight once you get to LAS.

Hope this helps!