Here in southwestern Ontario we don't typically see the kind of very cold temperatures you guys get in Edmonton, but we quite often see -10 C or colder, especially in the overnight or early morning. I never use 'Auto' climate mode in my eGV70, preferring to set everything manually. In winter I set the driver's side air flow distribution to all 3 directions - windshield, face and floor. The passenger side air flow distribution is always set to the only 2 options available - face and floor. Generally I never need to increase fan speed to above level 5 or 6, and when I shut off the car I usually lower the fan speed to level 1 so that the HVAC system doesn't immediately begin blowing cold air at a high rate on vehicle re-start. I generally leave the Heat button activated when the ambient temperature is below about 10 C. When the ambient is above 10 C I typically turn off the Heat function and go with the seat heater and heated steering
wheel only - that's usually enough to keep me comfortable.
The auto defrost mode you speak of is referred to in the owner's manual as "Auto Defogging" - ref. page 5-122. This feature can be enabled or disabled via the infotainment system in Climate Settings. In recent days I've noticed auto defogging activate several times in my eGV70.
I only ever press the stand-alone defrost button when the windshield needs major defrosting, with most of the air being then diverted to the base of the windshield.
As info, I usually set the cabin temp. control(s) to 18 C in winter.