1) When you set a signal to one-way it means the trains are forbidden to cross the signal in the opposite direction.
I usually don’t use one-way signals, as using two normal signals, one in each track will automatically make the path finding goes one direction and back another direction. One-way is only if you really want to forbid a train coming the opposite direction. Kind of useless with waypoints.
Sometimes the train ignores the path when you send it to depot for example. In this case, one-way signals are useful.
2) Yes exactly. If you have more trains is wise to divide in more blocks. The smallest blocks will allow more and more train, stopping and waiting less.
Problem with too many signals is that it doesn’t avoid trains being too close to one another, which is case of passengers is a big problem. Signals help to sync trains, to make them for far apart and thus more synchronized.