To the point number 3:
The vehicles disperse automatically on the line. If you add vehicles to a line, some of them wait longer at stations than others to get the perfect space between other vehicles, but this happens not immediatly, it takes some time (years) until they have all the same space between them.
The frequenzy is displayed in the lines overview (you can see this at several points in the gameplay video). You can shorten this frequenzy by adding more vehicles to the line and vice versa.