1. Yes, and indeed it may be preferred if you are referring to the group as being undecided, rather than its members individually.
2. "Has". "Have" would refer to the computers rather than the line.
3. "Are". The people are on the street, not the number.
4. Well, this is somewhat like #3, but there are contexts where the verb should be singular, where the focus is on the number rather than the individuals e.g., "A large number of people is expected in the hall, which has a capacity of 1,000 seats".