connecting in SEA is generally not a significant issue, although there's a good chance that the connection will involve the train between the North Satellite (AS) and the C/D gates (AS/AA)

during summer cruise ship season, particularly Fri/Sat/Sun mornings, the TSA lines are atrocious ... I'd strongly recommend against an open-jaw that involves an overnight in SEA, though if you decide to go that route I'd suggest that AA's late-morning nonstop to PHL might be a better option than the ~0800 on either AA or AS (although you may be contending with a lot of pax on international departures around noon)

I take a fair number of Sun night redeyes, and I've seldom seen long lines for those departures