I seem to recall when selling cars the V5 transfer also has a mileage (so easy to attribute)

It isn't precisely easy (MOT and tax timings won't line up etc) and arrears rather than advance etc. We definitely have enough data to do a fair approximation - just high operational overheads to collect

That depends. Do you also tax when the car is driven outside the country?

It's legitimate to track entry and exit on national borders though. A tax exception based on entry/exit times is doable and better than constant geo tracking.

It's utterly insane, but there is no equivalent to the MOT in the US. There's an emissions test, but not a safety test. I don't know if mileage is recorded at the emissions test.