hey again. you are welcome, however your info on power button is, well…
A simple momentary SPST pushbutton wired to pins 5 & 6 are all you need for a power switch, and a bit of code to monitor the pin. First press generates a proper ‘shutdown’ call, and once it’s down, the next press generates a reset. Well, yea, technically that is “additional hardware” but not like a new board to purchase. Some people worry about I2C being on those pins, but they are not electronics-minded individuals. You are either starting or stopping, so it will hurt nothing, in fact pulling a high line low is exactly how I2C communication occurs. :-)
Again, I guess your robustness to “power outages” will simply depend on what you want and why it matters. I mean with a big enough capacitor, say a 10F … not 10uF, but ten farads… (think coke can) or 100F (think Foster’s lager can)… is essentially a battery backup. One passive component. Although, massive. The competitive car stereo crowd in America loves these, because their ludicrous amplifiers pull more amperage than a MIG welder, and put such a strain on the vehicle’s electrical system. You can find them on eBay etc for cheap. I think Adafruit is selling a 700!!! Farad 2.5V, imagine a quad of those monkeys! You could run your pi with an LCD and a ton of USB peripherals, for days… no battery.
Point being, there’s many options. If it’s just about preventing corruption on your sdcard from intermittent power issues, just moving the running OS into RAM should be plenty.