Brr. Since you mentioned a diesel heater, life will be a lot easier with one of those, especially with workplace charging. That said, with -10F and the stock heater on MAX, my worst range was 40 miles/64 km.
At temperatures that low, you'll need to keep the battery warm. With a cold battery, you'll have an immediate 10-15% hit on range due to weak regen, and if it is too cold, won't charge. I definitely recommend using the slowest charge rate that time allows, and starting charge immediately after driving. Besides preventing lithium plating in the cells, a slow charge keeps the cells active and therefore warmer.
The often-cited battery thermal mod is removing a servo on the ductwork leading to the battery. This is found in the Driver's side footwell behind the USB port, at least on NA models.
https://www.dropbox.com/s/8xzriybp8ztvaxc/20140917_181508.jpg?dl=0
Unplugging and removing this servo allows one to flip the damper and send heated or cooled air to the battery at any time. Just make sure that the damper is to the battery before charging. I leave mine there full-time so I don't have to worry about it. To send air to the battery, just set the airflow towards the floor (feet) position. If you're sending air to the battery, I recommend using fresh air (recirculator light off). Besides putting a negative air pressure in the cabin, sending warm, moist air into a cold battery can cause condensation inside the pack, which is not a good thing with circuit boards being in the pack.
