An inversion of polarity would cause the destruction of the zener and would saturate the mosfet with which the line of 15 volts would obviously discharge the DS current that would support the mosfet would be destroyed so this circuit serves to protect the 3.3 and 5 volt lines against polarity inversion and prevents the voltage of 15 volts from entering deeper into the circuit. Unfortunately, here we don't know the value of the internal damper due to the destruction of the original and it is the greatest uncertainty due to the value of 1.6A. I would have to analyze the amperage of the circuit. The forward value of the damper at 1 volt is fine, I would adopt this mosfet I think so. because if it exceeds 5.3A it is destroyed.