What's stopping you from using an 8 MHz Pro Mini variant powered straight by 3 x AA/AAA cells?
Wow! You certainly have some knowledge and smart ideas. What's stopping me at this point is that I'm too far along in the project to go back and make major changes now; though if I'd tried the 8 MHz version in the beginning, it might have made a number of things easier, like you said.
I need the higher voltage to cover the voltage drop in:
(1) the protection diode I'm using in case the customer tries to plug the battery in backwards. (I made that mistake myself, which destroyed the whole board. It took three days for me to solder up a replacement.)
(2) the 5v regulator needed by the analog chips on the board (helping to isolate them from digital noise).
(3) the slow fall of battery voltage as it gets drained by use.
With my present setup, the battery lasts about 10 hours of continual use, before requiring a recharge.
The predicessor to my current project used two AA batteries, which I found uncomfortably heavy. THe current project used one Li Ion battery, the lightest of all rechargeable battery types.
On the topic of "The right FET for the purpose" the two FETs I ordered are now working perfectly for me. They are:
VP2106 (P-Channel Enhancement-Mode Vertical DMOS FET)
VN1206 (N-Channel Enhancement-Mode Vertical DMOS FET)
They probably wouldn't work in a 3.3 volt project, because of the higher Vth (threshold voltage) requirement (as pointed out above). I chose them largely for the simplicity of soldering the T0-92 case. (I couldn't even SEE the leads on those "ants"!)
Also since my posts above, I've discovered controlling the FET's by the Arduino's "brownout" function is not a good idea. The "brownout" function in the Atmega chip is turned off by default, and turning it on is a complicated advanced-programming process that cannot be reached by a simple "sketch". So instead, I'm including the line:
in setup(), which makes the range of the analog inputs reach from 0V to exactly 1.1V independent of supply voltages. Then measuring the center point of a resistance bridge between B+ and Ground (consisting of a 10M Ohm resistor and a 100M Ohm resistor) tells the sketch when to shut off the power to the whole board, via the FETs.