As for what to expect: the Q400 will park near the S concourse at SEA. Usually we park over by the Delta hanger and have to take a bus across to the S concourse; occasionally we park right at the S concourse. The bus won't leave until all the gate checked bags are delivered, so you'll have to wait for that. Then there's a long ramp up into the Customs area. If you arrive shortly after a long haul widebody, there may well be a very long line before you go down the escalator into the Immigration area; with NEXUS/Global Entry or Mobile Passport, you can skip that line to go down the escalator, but it's not always clearly signed. Best case is that there haven't been any widebodies arriving recently, in which case the 76 people on a full Q400 don't make a dent in SEA Customs capacity and all the lines will be short.

For re-clearing security: there is a dedicated security line just after Customs that regular departing customers can't access. It's often pretty slow and there's no PreCheck. If you're luck and get PreCheck, it is often faster to take the train as if you were leaving the airport and then go to a regular PreCheck queue in the main terminal. If you don't have PreCheck, definitely use the after-Customs security; SEA regular security is typically truly awful. There is a bag drop to leave any checked bags after Customs/before security.