3) Can you point me to any posting that shows that any professional mine has heaters in place?
I have a conductive paint on heater that is easy to put on a piece of wallboard or the floor and put on a thermostat. 750Watts would keep the container up to temp even if miners go off for hours. You could set it up to alert when power up and set temp to reach running temp prior to bootup to protect units.
I make this stuff really cheap and all you need is a thermostat take up no space as you could mount it on a wall or in the floor. Temp control should not be difficult if you are dealing with wattage for mining already.