Wow, you guys are awesome. Thanks for the great info and ideas.
This was the key I was looking for from BirdTravels: All ashore is 9:00-9:30. Once the last passengers are off, the shuttles stop. That was a tough internet search.
I think the PortValet is the way to go (was going to do that anyway). Really smart of Seattle to do this. Since the flights were booked by NCL I would think the airline (Delta) would qualify. So I'll cancel the transfer and do my own thing. Thanks again, and thanks for the links bluesea777.