reduce battery consumption

Should I concern that making very low power consumption impossible to achieve during sleep mode?

What happen if I am using the 8 D of alkaline batteries (total voltages is 12) external linear voltage dropped to 5 voltages output into the Arduino Nano's Vin, old boot-loader of ATmega328p, to stay power on during the sleep mode. Would this reduce power consumption or not?

Would you recommend me to replace microprocessor to Arduino Pro Mini? Because I discovered that on website Sparkfun it states that , "we can reduce the supply current from 15mA, down to just 4mA"

Pro Mini

I can make diagram on Fritzing if you would like me to. thanks

Please study this excellent tutorial on low power operation.

Almost everything you need to know is in there!

The Arduino Pro Mini can be easily converted into a "bare bones" equivalent by removing the regulator and the power LED with a swipe of a hot solder pencil tip.

What is powered by 12volts ?
For minimum quiescent power consumption, it is usually best to have a battery voltage of less than 5 volts, use a barebones Arduino which sleeps when not in use and, if you need a higher voltage, switch on a boost converter when required, using a mosfet (probably in a high side switch configuration).
You could also use 6 volts and put a diode ( say 1N4007 ) in series with the power to the Arduino, but the boost converter would be powered directly from the battery. Since you are using separate cells, you could simply take 4.5 volts between the third and fourth cell and dispense with the boost converter.