1. I tend to stay through bad movies, but I will walk out if something is seriously wrong. The only time I've ever walked out on a movie was during Transformers 3, which was a bad movie in its own right, but that wasn't what made me leave. I couldn't stand what it was doing to Alan Tudyk. I left because of that. As for emergencies, etc., I think that if you have a scenario where you would need to be called away at a moments notice, you really have no business being in a theater. People say, "What if it's a doctor that needs to be called away?" Easy- if you're a doctor on call, what are you doing in a movie theater? If you're not on call, and for some reason, you get paged, at least have the decency to leave quietly. I'm not completely unreasonable about it.
2. I always stay through the end credits. I like listening to the music, I think staying gives you a few minutes to think about the movie, and I like seeing which sections were larger than others. For example, I just saw the Impossible, which had a veritable army of crowd hairstylists. It's also nice to respect the people who worked on it. Even if it's a bad movie, the crew was probably doing their best.
If I bring a date, though, I usually won't make her stay through. Most people aren't used to it and if they don't like it, five minutes of boredom is enough to kill any mood. Sometimes I'll try to make it fun by making fun of people's names or titles, but it's pretty disrespectful. Oh well. I think they'd understand.