in answer to the question of free available pins , no it s full house , i would love to the only input pin , but its not able to do adc ( vpp/m-clr) , as far as the math i see it as this

input Voltage range at regulator to produce output of 3.3v = 3.8V - 7V
low voltage monitor trigger point - 4.5v ???
Pch mosfet gate threshold highest value = below 2v will most likly be between 0- 1v to turn on mosfet ?
Pch mosfet Vgs greater than +-5v - better at -+10v for safe use

any thing i missed ??