PWM low frequency generation low current draw
The device must work on two 9 volt batteries in series... 6.94 mA
  isn't enough to power my Arduino Nano on sleep mode.


The Arduino Nano has a linear regulator so powering it directly from 18V is extremely inefficient and a bit dangerous (absolute maximum input voltage of the LM1117 is 20V). With 18V in and 5V out the on-board regulator drops 13V so it is only 28% efficient, wasting 72% of the battery power. 

However if you use an external switching regulator (aka 'DC/DC converter') it will increase output current as it reduces voltage, typically with 70~90% efficiency. For example, this buck converter module converts 18V at 3.75mA to 5V at 10.6mA (measured values), corresponding to an efficiency of 77%.

But you may be able to do even better than this. The Nano has a USB interface chip which draws several mA. The Pro Mini has no USB so you have to program it with a TTL-USB adapter cable, but the current draw is much lower. The 5V (16MHz) version only draws 3.2mA in sleep mode, but most of this current is consumed by the power LED and 5V regulator. If you remove them it only draws 0.06mA in sleep - less than the quiescent current draw of the DC/DC converter above (~0.28mA).  

With more advanced tricks such as switching to the 128kHz internal oscillator or using a 32.678kHz crystal it could go even lower, then the only problem is finding a regulator with low enough quiescent current to make it worthwhile. In one project I used the LP2950 which has a quiescent current draw of 0.075mA, low enough to be insignificant in my application.