Why have both a Schottky Diode and a MOSFET for reverse voltage protection?
The MOSfet is not configured for reverse polarity protection.
It is arranged so that it will turn OFF on over voltage. So it is over voltage protection.
At voltage below 16V the MOSFET will be turned on by bias through R4.
When the input voltage goes over ~17V the transistor Q2 turns on by current through the zener diode D3. Q3 takes away the bias voltage for the MOSFET and turns it off to protect the following circuit.
The MOSFET does not do reverse polarity protection because of its internal body diode.