I don't think you're going to have a problem on Tuesday. Most people vacationing will go back on Monday. So it would just be the Sasquatch crowd and potential traffic if you end up in the Seatac area during the morning rush, which gets better around 8am, so you should be ok. I would allow 3 hours to get back to be safe and take the hour and a half early at the airport seriously.
That might be tricky, a lot of people will want to be there before the bands start on Monday, but they will still be trickling in after noon. Plus, you have to consider the holiday traffic on Monday. If you are going on I-90 towards Seattle you shouldn't have a huge jam until you get close to Seattle, which can be rough. However, since I have not camped I don't know how getting in and out works, can someone else speak to that?