Using re-circulation is the important thing. I don't know if Auto uses it - I'll have to give it a try. (Most auto-whatever algorithms don't do something I want so I rarely use them.) You want to heat yourself, not the outside world. Constantly heating 0F, 20F, even 40F air up to 70+ requires a lot more heat than just keeping the car warm using inside air. But you have to keep an eye on it because the car will periodically switch recirc back Off. I think when its cold enough it may stop doing that. Set MyView screen to show ICE temperature and watch what happens. You can be driving non-stop 'highway" at moderate speeds and without recirc the ICE won't stay up to the minimum temperature needed to center the gauge. Recirc is also a must for AC. Of course there are times when there's too much moisture in the car and you need outside air - but not likely when its really cold.
For defrost I just turn it on as needed and don't worry about it. If you watch Climate Power in MyView you can see when the AC compressor is running during defrost. As I recall, power draw is quite low - or zero. You may have to turn off recirc. A clear windshield is a must of course.