Sounds like a nightmare.
Regardless of who the retailer/seller is, with dash cam evidence etc - small claims court is the route should be considered.
"2000 miles test drives,"
At times, I video my mileage when handing the car over to a garage as this date stamps and times it - just a simple backup along with the signing in of the car and out - just in case there are fines/toll charges against the car when the car was not with me.
Call me sceptical, but I don't trust any garage as the garage/retailer may be very decent but it only takes one employee to mess things up. Having said that, when things go wrong, a good garage/retailer with a good manager will quickly and politely sort it out.