If they’re not moving and have a heat pump, well, but like a combustion vehicle will exhaust their energy storage eventually. Resistive heat or suboptimal battery architecture? Poorly.
Jerrycans for some, tows to Fast DC chargers for others. Class 8 semis should have sufficient diesel reserves for long loiters, even with truckers using auxiliary power units for heat. If not, it is trivial to refuel them on the road with a transfer pump.
This is a misconception. Heat pumps can improve things a little but often far from as much as people expect. The problem is that in cold conditions most of the loss is from reduced battery performance right from the start. In really cold conditions it can be 50% and more reduction in battery performance. This is energy you're not getting back, heat pump or not. In addition the performance depends on the temperature. There is a sweet spot and the colder it gets from there the less effective it becomes. At around -20 degrees C and below it's effectively useless.
There are already vehicles that have been tested in real world conditions. Tesla Model 3s exist with and without heat pump. The difference isn't that large, around 10% more range with the pump in perfect conditions. I've seen people doing tests sleeping in their cars all night and in cold North American winters it's not pretty. With a fully charged battery you will get enough heat for a comfy good nights sleep, 8 hours or so. So lets say in an emergency situation you can stretch it to 12 or so by reducing the temperature. But that's on a full charge! Most cars getting stuck in traffic are not going to have fully charged batteries. It absolutely is a problem with EVs even more than with ICE cars.
Aren't EVs programmed to hold a reserve charge for emergencies? To avoid the cost and inconvenience of a tow, shut off power before you lose the ability to drive yourself to the nearest charger. (Bonus points for adjusting the threshold based on the distance to known stations.)
But in the case that a car does drain the battery, how difficult is it to get a portable charger to someone? Have roadside mechanics started carrying generators in their trucks now? They probably should, with the appropriate cables.
To your first paragraph, yes, but battery charge can decline rapidly in the cold at low states of charge. You may exhaust the reserve depending on your circumstances.
To your second paragraph, AAA (the tow service) piloted mobile generators for stranded EVs. There was no demand, and the service was discontinued.
Jerrycans for some, tows to Fast DC chargers for others. Class 8 semis should have sufficient diesel reserves for long loiters, even with truckers using auxiliary power units for heat. If not, it is trivial to refuel them on the road with a transfer pump.