If one is looking to leave SEA to east coast on a lie-flat and doesn’t want to double-connect, probably much better to look at DL. Easier and likely cheaper on any given day as well.

Leaving ex-YVR is fine…if someone has time and wants to make effort, plus, almost certainly more expensive. It’s either a double-connection that is illegal cabotage, or better than flying SEA-YVR, it’s probably easier to just get there separatelya d buy air ex-YVR. There’s Amtrak service (both train, and they at least to market their own bus trips), as well as a SEA airport to Vancouver bus (can’t remember if it goes to YVR airport or just downtown).