The Pfet is a great solution for reverse polarity protection, but as I said it conducts both ways, so current can flow either way, as long as the FET has gate drive.
To use it for reverse protection, you actually connect it “backwards” so the body diode is conducting when in the correct configuration (i.e. source and drain reversed from what you might conventionally expect).
E.g. see http://www.ti.com/lit/an/slva139/slva139.pdf
Page 7. For a more detailed explanation of how it is typically used in e.g. automotive electronics.
Problem is, for e.g. OTSM, is that, unlike a diode, the Pfet will continue to conduct in either direction as long as it has gate drive, meaning that in a typical torch, even when the drive to the LED is disconnected by the MCU, there is a “reverse” (actually forwards, in the usual sense) current path back through the Pfet, which will instantaneously discharge the OTSM cap into the LED. Whether e-switch or clicky. Until it ceases to drive the gate.
I haven’t thought about your high-side drive arrangement, there may be some better possibilities with that.
My only concern about using a simple Schottky instead is because of the concerns with previous Attiny versions, in standard, and selected low voltage versions, and the experiments with BOD, possible flash corruption whilst saving settings in particular circumstances etc. Where a few more 100 mV supply might help.