I'll take on the first question. Yes, it is indeed used as reverse battery protection. The MOSFET will conduct in either direction when turned on. With the gate grounded, current will flow through the MOSFET body diode to the source, and when the voltage gets high enough the transistor will turn on, bypassing the body diode. But with reverse battery installation, the transistor remains off, and the body diode also blocks any current. This is a pretty standard method of reverse polairy protection, and it's used because in normal operation there is virtually no voltage drop across the MOSFET, whereas a diode would of course drop some material fraction of a volt.