As a slight aside, if you wanted to see examples of what happens to temps when there isn’t enough heat supplied you could see the extreme version of that where people are completely turning off their heat pumps:
That includes some graphs showing how the initial heat loss is notable but then the equilibrium is reached as mentioned.
However, in your scenario the delta would be significantly higher so the house would cool further than those examples.
In my personal experience of -16 °C for a few days the 14kW oil boiler struggled to keep the house warm. Our 14kW heat pump would struggle too. At least the icing up would be less likely because the humidity at that temperature is much lower. Clearly the solution is to improve our insulation rather than fitting an even bigger heat source. If we had lost electrical power the oil boiler would have failed so we’d be in the same (cold) situation as with a heat pump.
As you imply, having the entire country running their heating at full pelt would be stressful for the grid. Others have mentioned this and are working on that problem too. It might be interesting for you to read this consultation which refers to other heat sources such as district heating (which may be heat pump or other types):