How to determine resistor value for PWM signal?
The answer to all four questions is that you should not put a resistor there.

That resistor shown in the picture is not an external resistor that should be present in your PWM driving circuit. It only tells what kind of a load the fan presents to your signal, i.e. it is the PWM pin input impedance. The output impedance or driving ability of your PWM source determines how many fans can be connected to it. Of course you can add transistors or logic buffers to drive more fans.