Zener diode as a Voltage regulator, where do the formulas come from?
All of your assumptions are entirely correct. The piece that you are missing is when calculating Rsmax it is assumed that Iz is zero. The idea is that Rl is using all of the available current and the Zener diode is not conducting at all. If the Zener current is zero then it follows that the circuit is a simple voltage divider. The first equation that you have is the voltage divider equation that is solved for Rsmax because the other three values are known.