Reverse polarity and reverse current protection for battery charger
This circuit will work, but you have to be careful selecting the PFET to make sure that you choose one with a very low gate threshold voltage to make sure it is fully on at the lowest operating voltage.  Remember that with a one cell battery, Vin might be much lower than 3V for a deeply discharged battery.

http://www.ti.com/lit/an/slva139/slva139.pdf

To answer your other question, when the battery is first connected and the buck converter Vout is 0V, the PFET will be off and the charger will not be able to sense the battery.