If you need to travel book out of SEA. There is no point in going through contortions getting onto a flight out of PAE when the chances of it operating are highly unlikely. If you want to travel for fun from the new airport, you are going to have at least one month notice from the time the government opens to the time it will even remotely be possible for PAE to open. The longer the government is closed, the longer the lead time to open it.