I was recently admitted to NU, and while I'm hardly an expert on the program, I've been doing a lot of research lately that's relevant to a couple of your questions.
I don't know much about USC, so I'll just answer these from a Northwestern standpoint.
1) At NU, shows are split between TI (faculty-run mainstage) shows and StuCo shows, which are entirely student-produced. There are about a dozen student theatre groups and tons of opportunities to get involved. I don't know the exact number... but StuCo has their own wiki that looks like it could be a fantastic resource for you. StuCoWiki
4) Creeping on the NU Theatre Class of 2013 Facebook group, I saw a TON of film students posting audition notices for films they were making, which suggests a good relationship. Theatre students and RTVF (Radio/TV/Film) majors are both in the School of Communication at NU. At USC, I believe theatre students and film students are in separate schools within the (very large) university, so the relationship may not be as tight-knit. But again - I'm not a current student at either of these schools, and I'm inferring all of this from things I've read.
6) About 30-40% of NU's student population goes Greek. Since it's a pretty prominent part of social life on campus, I'd assume at least some theatre students participate in Greek life.
8) Northwestern has many respected faculty members, especially in the Chicago theatre scene. Example - if you've heard of the play August: Osage County, the director, Anna Shapiro, is a professor of theatre at NU.
10) After talking to some current Northwestern theatre students, they all love that NU has the "best of both worlds": a very intensive theatre program that still allows for a fairly rigorous liberal arts education since it's not a BFA. A lot of kids apparently double-major or minor in other things. Another strength of Northwestern is its proximity to Chicago, which is obviously an amazing theatre town. NU theatre students work at Steppenwolf and Victory Gardens and the Marriott even when they're still in school.
All of this said - I've heard FANTASTIC things about both programs, and it's really just a matter of which school feels right to you. Congrats on your great options, and good luck making your decision! It's a win-win situation, as far as I can tell.