I’m presuming you’ve leased the car from Motability (not bought it) as you mention ordering another.
Unfortunately there’s no way you’ll be able to order another Mercedes from Motability.
If the hire car isn’t suitable, i.e. your disability makes driving the car difficult, painful, unsafe, etc, then Motability should offer a different hire car. But not being your personal preference will not be taken in to consideration. E.g. you can’t demand an hire car that matches the spec and desirability of your current lease. This will be frustrating as you agreed to forfeit your benefit for 3 years for a Merc and that’s not what you are driving. You will of course, be able to cancel your current lease, but will not have a vehicle.
Motability may be able to source a vehicle soonish for you, but it will obviously be a vehicle that is in extremely low demand (a car not many people want).