I think you have a pretty good chance. I applied, got into, and turned down UW last year. I had a 3.78 uw GPA and a 1740 on the old Sat (equal to 1260 on new SAT). You have really good extracurriculars and a pretty good GPA/SAT score. Be sure to focus on the writing section of the application, and I think you have a pretty good shot at getting in.
Calm down, it's one quiz and it's not like you are failing the class. A B+ is not going to prevent you from going to a good school. Once in college, things like this will seem really trivial and unimportant.
Since you're putting off work for a little bit, I would try to focus on joining some on campus clubs/organizations, find people in your major, and go to different campus events. Maybe try to find others to study or do homework with.
I would say that you made a good decision. If you plan to spend a lot of time in your dorm studying, eating, etc. then you want to be comfortable in your room. Trust me, the 1 bathroom for 8 people situation would have gotten old really fast. I think it is very important to be comfortable in your dorm, and that your decision reflects that. Give the new roommate a chance and if there are problems, have the RA help you figure things out.
Percy Jackson series, Harry Potter series, The Great Gatsby, Frankenstein, To Kill a Mockingbird, Everyday by David Levithan, and any book by Mitch Albom. Those are just some of the few that I can think of.