1: Sofa vs. couch: I'm sure that people who work in furniture stores will tell you there's a difference, but I don't think there is one. I use them interchangeably.
2: As MH says, not necessarily. But they can be.
3: Yes, two people. (More can squish in, of course, as you would know if you ever had six nine-year-olds watching the same movie.)