Don’t forget that your contact is with Motabilty, not the dealer, so I doubt you will get anywhere without involving Motabilty.
As above, by putting your PIN in on the day of handover, you are entering into a legally binding contract to accept the car.
On the other hand, your wife has a medical condition which means that life would be considerably harder without the feature you thought you were getting, so do contact them explaining the situation and hopefully they may show empathy and help you to get the feature retro fitted.