and/or using a high-side P-MOSFET
It is a bad idea to use 12V as input to an Arduino, when you are powering external devices with the Arduino regulated output. The on board linear regulator wastes an enormous amount of energy and overheats.Unless you really want to learn the hard way, don't waste your time. Use an external switching regulator and a Pololu power switch, as recommended in reply #3.
While I wasn't ever planning on using a 12 V battery
Glad you did those calculations and learned something.Considering the above quote, please explain why you posted a diagram, clearly showing a 12V battery power supply.