Tesla has made the heat pump system fairly complicated, at least according to a patent they filed. In the cabin is both a condenser and evaporator, so yes the single heat pump can both heat and cool at the same time to deal with humidity issues. (
@mongo has made several posts with details about the patent in other threads.)
Tesla has designed a number of strategies to deal with extreme cold. The heat pump can scavenge heat from the power train, it can run the heat pump in an inefficient mode to create heat (like how it heats the battery in the Model 3 by running the drive units inefficient), it can run the HVAC blower inefficiently to create extra heat, and finally there are two LV, i.e. 12V, heaters in the cabin that can provide heat. (Though from the patent they are mainly used for when the driver and passenger have different temperature set points.)
So it seems like they have all of their bases covered. Hopefully it proves to be reliable.