Designing reverse and overvoltage protection at 3.3V
The simplest way to implement reverse polarity protection is to put a diode in series with the PTC fuse.  If the voltage on pin 1 is negative with respect to ground, the diode will no conduct, and the circuitry downstream won't see the negative voltage.  Make sure the reverse voltage rating of the diode is large enough to handle any negative voltages you might expect to see on the input.

During normal operation, the diode will drop some voltage, but you can find low forward voltage diodes, Schottkys for instance, that can get down to the 300mV range.  If you can handle the voltage drop, this is probably the easiest solution for reverse voltage protection.

As for overvoltage protection, as Sunnyskyguy said, changing the input voltage to a higher voltage (5-12V), and regulating it down to 3.3V is probably your best bet.  I know you said you don't want to add in a separate power supply, but any overvoltage protection circuitry is probably going to be as complicated as adding an additional voltage regulator.  There are plenty of other benefits to using an on board regulated supply, rather than relying on a wall wart to generate your 3.3V rail.  You can have a large input voltage range, you are less sensitive to series voltage drops (from the PTC, from a diode, from the MOSFET, ...), the input current is less (assuming a switching regulator), etc.

Are you locked in to using a 3.3V wall wart, or is it possible to use a higher input voltage and step it down on board?