Technically you don’t have to accept the vehicle…
You purchased with parking sensors in the specification I assume. The dealer messed you around a couple of times and sourced a vehicle from stock at another dealership. That vehicle was not the spec you ordered so they are retrofitting sensors. That’s not what you ordered and the entering of the pin happens way to often before seeing the vehicle, motability are quite understanding in this area.
I’m sure if you explain the experience to motability, and with the car not even being the correct spec they will understand. Have a think about your preferred option as I’m sure you have at least a couple to choose from. Sorry to hear of your bad experience.