Don`t forget that if you cancel on the day of collection the car will have been registered with a number plate and would then be classed as second hand so they could claim the the difference in new and second hand price but it is unlikely.
Dealers can DE register a new car, but it has to be done within a few days i think before adding the extra keeper. or like you say they’ll have to sell it as second hand or ex demo.
Again if its mis-sold then it would be the dealers fault anyway.