Whether or not an outdoor heat pump coil will freeze or not depends on the dew point temperature of the air passing over the coil, and the surface temperature of the coil itself. If you're really curious about this, monitor both the dew point temperature of the air entering the heat pump, and measure the coil surface temperature. Ambient dew point temperatures can become quite low when a large, cold air mass (called a cold front) invades a region. In spite of what may appear to be high relative humidity readings during cold weather, when this high RH is translated to dew point, the actual moisture content of this same air can be quite low.
As for the crankcase heater on the compressor, if you experience a sustained power loss at your house in winter (and you live there...it's not a vacation home), turn off your heat pump, turn all of your appliances off and unplug all electronic devices in the house. Drip all indoor faucets.
There is no guarantee that when power is restored it will be clean, full power (i.e. "brown outs", etc.). Provided it is, place the thermostat for the heat pump into emergency heat mode. The electric heat strips will begin warming the house while the compressor crankcase warms up without running.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.