Attire is semi-formal to formal. The average temperature in May is 60s in the evening to 70s and 80s degree during the day.
Public transit is decent. There's light rail, buses, Uber, Lyft, and cabs. I don't recommend renting a car because parking and traffic is horrible here.
There is street and garage parking, but no parking directly associated with the venue.
Both the ceremony site and reception hall are located on University of Washington Seattle campus, which is in the University District.
Any hotel in the University District will be close to the venue, either by walking or a short uber/taxi ride away. Downtown is about 15-25 minutes away (depending on traffic). The light rail connects downtown to the University District, which is the better way to get around between the two location, should you choose to stay at a hotel downtown. The light rail takes less than 10 minutes to get downtown. I also recommend looking into Airbnb (lots of good options in the area).