I’m going to venture the following answers based on having a daughter at AU who is in her first year:
1 I wouldn’t call it preppy as a whole but I’ve seen a little of everything so that I wouldn’t imagine a prep would feel out of place. And I think it’s fair to say the dress code is more towards preppy than towards artsy or bohemian.
2. It’s pretty known for that General area, including public policy and international affairs.
3.my understanding and observation is that AU is pretty liberal both politically as well as socially but as to whether or not someone conservative would feel uncomfortable, I can’t say. Hopefully someone else can weigh in.
4. Those grades should certainly get you accepted regular admission but I’ve no experience with ED, sorry.
American University is a very liberal school. The DC area is quite liberal in fact. Most colleges are liberal, if you are serious about avoiding liberals or “non preps” you should look into religion affiliated schools like Pepperdine or Boston College