Interview Friday, help appreciated

I have an interview Friday for a summer audit position with a Big 4. I waltzed into my last internship through a quick connection, so this is the most formal interview I've had to date. I'm absolutely terrified due to a combination of inexperience and a burning hatred of behavioral interviews, and we've already been informed there won't be a technical portion. I'm sure I'll have a ton of questions as the week progresses, but I wanted to throw the first two up as soon as possible.

First, how open should I be about the fact that I don't want to spend my whole career in public accounting? Should I make it sound like I'm gung-ho audit for life, or should I say something reasonable about how I'd like to shift to something without a busy season as I start a family? I'd imagine that's something they're perfectly used to and it shows that I've done some homework on what different careers in accounting are like, but it has a rather obvious downside. For what it's worth, neither of those are true, but my real reasoning isn't something I'd blurt out in an interview and 90% of what I say will be bullshit anyways.

Second, they're having a mixer Thursday night. Am I completely fucked if I don't go to this? Because I'm 90% fucked if I do. Another Big 4 was on campus recently, and I was invited to an info session. I made the fatal mistake of showing up 15 minutes early and they were essentially having a mixer. I don't know a single person in my class and I'm awful at meeting people, so I stood in a corner the whole time and needless to say, got an e-mail that I wouldn't be getting an interview. Is there an excuse that would get me out of this without raising too many eyebrows? Hard to say I'm out of town since it takes place the night before the actual interview.