High-Side P-Channel MOSFET Won't Turn Off!
On reverse polarity protection:

In the schematic in the question, Q1 still conducts in case of negative input voltages (= reverse polarity) and D1 creates a shortcut. I assume, that this was not intended. 

Q1 must be connected the other way round like this:



The Z-Diode is only necessary, if the maximum input voltage (+BATT) can be greater than the maximum Gate-Source-Voltage of the P-FET (usually 20V). [The 47k in my schematic can also be 100k. This should not matter.]

On positive input voltages the internal body diode charges the capacitor. The voltage over the capacitor turns the P-FET on (Vgs must be negative; gate must have lower voltage than source). On negative input voltages, the internal body diode is blocking, thus preventing reverse polarity from doing harm.

On LATCH:

If C7 charges up, Q4 is turned off. So, R7 and C7 must be interchanged. The polarity of Q4 in the schematic in the question is right. 

These points should fix your problems. 

Do not worry about defective FETs. There are no defective FETs shipped. All of them are right, when delivered to you from the original manufacturer. If you find one, that is defective, assume you broke it and look for the reason. 

Good luck with your circuit and have fun!