How to add a MCU signal to turn off this push button mosfet-based latching circuit?
I would use a bipolar transistor with its collector to the right terminal of the on/off switch.
Emitter to ground. And a resistor to the base that is driven by an output from your MCU.
Set the signal high to turn power off.
Ensure that the transistor does not get turned on during the power-up and initialization of the MCU.