1) If Arduino gets too little power I wil not do anything so it will never get enough power => stays "dead" but undamaged
Well it may not work (I would guess not, but for no specific reason) but not for that reason. His Arduino is getting power directly from the battery pack. The output of this proposed 'programmable regulator" is the blue wires going off the page, this regulator doesn't power the Arduino.
As far as if it will work, you can only experiment and see. I don't see any risk to the Arduino board as you are using a voltage divider for the Arduino analog input pin.
Art work is cute, but a real schematic would be a treat.
On second thought I'm sure it won't work. There is no voltage being applied to the gate of the mosfet from the collector of the npn transistor. This is a semiconductor fail.