which spikes the electricity demand from the grid and puts it under the most strain when its most critical for the grid to stay up, which means the grid has to be overdesigned, which means that the air based heat pumps are a poor choice for a reliable grid.
I don't think this conclusion follows. It is true that variability is a challenge for grids, but it is not clear that it is better to trade more total energy usage (per useful unit of work) for less variability. Variability is certainly a challenge, but not an insurmountable one, and also one that must be faced regardless nearly everywhere, as solar power has become too cost-effective to be ignored.
The UK is rolling out smart meters to every property. In the past 2 winters (following on the Russian gas crisis), they have run programs that pay £3 per kWh reduced demand. This is a nice way to balance the grid during extreme demand.
It's called the National Grid ESO scheme and you sign up through your supplier when it's open. However, some suppliers take a cut of the ESO payment. Alternatively, you can sign up to Uswitch Utrack (https://www.uswitch.com/mobile-app/), which passes 100% onto the customer. (Disclaimer: I used to work at Uswitch.)
It depends on where you are. In many places worldwide, extreme cold is also generally quite clear from clouds unless you're literally in a storm, in which case, the high winds are just as likely to cause problems. Rooftop solar should be more than enough to offset your own usage.
In nearly all places extreme code means dark! Sure there are less clouds, but the latitude is high and so there are not only few hours of daylight, the earth's angle is also working against solar.
