A reversed zener is just diode. It gives you a voltage reference, but it's less stable with current or input voltage changes.Can someone explain why it woks with reversed polarity with respect to the schematic?
The range depends on the feedback from the voltage divider on the output.I also suspect that with an even smaller zener, I could get even more range. Which also tells me I haven't understood the circuit, yet.
The purpose of R2 is to bias Q1 on; to the point of saturation without Q2. Q2 takes current from the base of Q1, but the current goes through the zener which is your voltage reference. Zeners have a sharper knee than diodes, so they provide a more stable voltage reference.
Without a zener or diode, you're using the BE junction of Q2. That won't give a very stable reference.