HI
Unnecessary to this additional circuit,if you want to have protection please share your circuit for review!
I'm just looking for information on whether the PMIC could be damaged if the coin cell was inserted backwards and the battery polarity was reversed.
Absolutely damage the PMIC if reverse the input voltage whatever the battery or VINx.
What if only the coin cell is reversed and no power is applied to VIN?
Also have damage risk!