Tricky to do accurately though. Like a car driven occasionally in the countryside may cause little damage from brake dust, a cab in a city center a lot.

Not necessarily: You get a mix of average representative driving profiles, drive that with a vehicle and measure how much particles are emitted. This is how exhaust emissions are already measured.

In practise the biggest danger is that manufacturers game that system like VW did by recognizing a test environment and adjusting accordingly.