2: Don't you want actual examples?
a: A car moving directly south at 2m with wind moving directly south along with it at 2m.
b: A car moving directly south at 2m with wind moving directly north against it at 2m.
c: This cannot happen.
They all look correct to me.
Though for the magnitude to be less than 4m you can say the angle between the two vectors should not be zero. Or the vectors should like at an angle θ.
Separate names with a comma.