Why not 3 AA batteries ?The cheapest crystals on Arduino clones are bad quality. They might break when an Arduino board is dropped on concrete. They stop working below 0 °C. If you put many Arduino boards in the freezer, all the cheap ones will stop working.When the voltage is low, they probably will keep on working. The circuit inside the ATmega for the crystal can be set to normal and "full swing". The normal mode should be good enough.Since they are encapsulated in a metal box, I think that RF noise is not a problem for the crystal.
The nRF was put to power down (with oscillator stopped) and then restarted again so it restarted even this cold. In there freezer is more cold than outside in winter. I did only short test to not stress the circuit (batteries mostly) more than needed. It may not prove anything but I hope it means there is a good chance it will work. I will put it in real life conditions and see results. Thank you for inputs.