1) Because that's how they do it in East Asia. (I'm from Southeast Asia, by the way, and in my country, we either use indoor slippers or just going barefoot in the house.)
2) It's an exclamation, sort of like "oh my" in English I think.
3) I guess it depends on your relationship with the other person. The more formal and/or impersonal your relationship with the other person, the more you are required, by common courtesy, to bow in acknowledgment. (Anyone can confirm/refute this?)
4) Dramatic license? And maybe it's okay when you're doing it among close friends and family?