If I were to err on the side of caution, I think I would switch over to the Arduino Pro Mini to take advantage of the lower voltage requirements and therefore the really liberal capacity of the lantern battery - 52000 mAh. The Arduino pro Mini also allows you to configure it to bypass the regulator. I would still need a regulator, but I think I could benefit from a LM7805 switching regulator instead of the linear regulator native to the Arduino Pro Mini. If I did all this, including your suggestion to remove the power LED, is it reasonable that I could get the power consumption to 25 mA - 5mA(LED) - 10 mA(Regulator) = 10 mA? And given the 52,000 mAh from the lantern battery, that I could get a life of 52000 mAh / 10 mA = 5200 hours or 216 days?