Are there restrooms?

Yes. If the plaza is open which it should be open 30 minutes past sunset there are 2 sets of restrooms on the parking garage level as well as on the 1st floor near the SAM Olympic Sculpture park entrance at the intersection of Western and Broad👌

Yes, they are located inside the parking garage, on the right hand side past the orange lockers.

There are not,but you can go to a restaurant nearby.

I didnt see any in the park but you can go to any close restaurant or shop

Not that I know of, but you can probably walk to waterfront and use the restroom in one of the restaurants