Is this ADC circuit safe for when my Pro Micro is off?
It's not a good idea to have voltages higher than the supply voltage on a microcontroller. The controller indeed has clamping diodes to protect the inputs against too high and too low voltages, but it's a Bad Habit™ to use them intentionally. \$\mathrm{CB_{SENSE}}\$ won't be higher than 0.67 V and you would think this is harmless, but on page 378 of the datasheet it says under Absolute Maximum Ratings: 


  "Voltage on any Pin except RESET and VBUS with respect to Ground: -0.5V to Vcc+0.5V"


So even that low 0.67 V is too high. Your gut feeling may say that voltage and current are so low that it's probably harmless, and I would agree if the datasheet didn't say otherwise:  


  "Stresses beyond those listed under “Absolute
  Maximum Ratings” may cause permanent damage
  to the device. This is a stress rating only and
  functional operation of the device at these or
  other conditions beyond those indicated in the
  operational sections of this specification is not
  implied. Exposure to absolute maximum rating
  conditions for extended periods may affect
  device reliability. (emphasis mine)


So, while you would want to take the risk for your one-off project I wouldn't do this for my 10k/year design.