Is this resistor necessary for protection?
R3 is the essential pullup to Q3 to turn off the MOSFET pair with R2 in series .

The purpose of this circuit is to replace a diode voltage  drop for Reverse Battery Protection with 2 series high current low RdsOn switches to minimize drop voltage during battery charging.

In the fine print it offers two different Zener voltages ; LV and SV because standard threshold FETs (2~4V) have an absolute max of |Vgs|=20 so the higher Vz is needed, and sub-threshold FETs Vr&lt;2V need Vgs=2.5Vt and lower Vgs Abs Max.

Not all FETs have the same threshold to switch on.  For a low voltage pack, a low Vgs(th) or logic level type FETs would be used and for high series cell packs a std threshold FET may be used. But they need 2.5 times their threshold to conduct near rated RdsOn and 3x threshold for std FETs (2~4V threshold)