15

OP, this is a ridiculously long thread for such a small subject, and in any case it's certainly your call, but I'm not sure why you're continuing to weigh alternatives when you already have a number of them that will work. The cruise ship terminal mentioned in post #1 doesn't require you to be a passenger to store bags, or so their website says. If you want to use a hotel, I think you'd be able to do it that way too. If you're willing to ask your hosts to backtrack to the airport, you can store the bags there.

As for the Experience Music Project, I wouldn't be surprised if they required you to enter the place as a condition of storing your bags. The place gets mixed reviews (to put it nicely); is somewhat out of the way relative to the part of downtown that tourists care about; and the admissions charge is $15 per person. If they say no, and you don't want to visit the place, you'll be kinda sorta stuck there because your baggage storage alternatives (the cruise terminal, full-service hotels with bell desks) will be a mile or more away.

If for some reason you are intent on storing your bags at a museum rather than in any of the other places that have been noted, then you'd be better of trying the Seattle Art Museum, which is smack in the middle of downtown and (unlike EMP) is well worth the "suggested fee" of $15 for adult admissions.