The way I’m seeing it is the dealer did not charge/invoice you for the extra’s, rear view camera and the wheels.
Don’t think Motability will have anything to do with this because MB says that optional extras added to a Motability car are between the dealer and the customer. As for paying up I think you might be on sticky ground if you don’t settle the bill regardless of it being their mistake.
Could be wrong about this but sure that’s what I’ve read somewhere on the MB site and looking at the Motability contract for my yet to be delivered next car there is no mention of the extra I’ve added to the car or there cost but I know they got to be paid for on collection.