You are free to work directly with the hotel to book the additional dates you need. The rates will be at normal prices however as the group rate only applies Thursday through Monday. You can arrange to stay in the same room through multiple reservations.
There will be $25 vallet parking. There should also be plenty of street parking available as well. However, if you're staying downtown, we recommend you Uber or take public transit. Both Pioneer Square or Symphony light rail stations are a 10 minute walk from the venue.
Cocktail attire is perfectly fine. As long as you're not showing up in flip flops and shorts, you'll be good to go.