Since you live "here" also then you hopefully understand the traffic flows (or lack of it) to and from Snohomish, King, and Pierce Counties into and away from the 2 main mountain passes (Stevens and Snoqualmies) depending upon the day of the week, time of day, and rush hours. You know the routine: the absolute worse times to be heading east out of the Sound into the mountains is Friday afternoon/evening and heading west back toward the Sound is the same time period on Sunday. Both mountain passes are even worse on holiday weekends.
Using the above as your guide you'll do fine no matter which way you decide to get to and from Leavenworth. Seeing as you say you're leaving Leavenworth on Tuesday, just try to time your travel through Seattle or Tacoma back to Belfair during non-rush hour. Good luck.