The mirrors are flat, and the sun has an apparent diameter of about half a degree, so at 100 km, the spot diameter would be 900 meters.

You could use concave mirrors, but since you're moving them independently, you'd also have to consider the diffraction limit for each one.

If only there were some extensive property of matter we could multiply with the temperature to compute a weighted average. That would be massively helpful.

How do you figure? Everyone who doesn't live in big cities has the ability to charge an EV overnight, or in half an hour when road tripping. Absolutely none of them live within hundreds of miles of hydrogen refueling station.