I would start by having a conversation with Motability. You will need to be with the account holder if they’re not making the call.
Run through the timeline of what’s happened and failed plus what’s been said by the dealership. The fault is sufficiently serious as the car is undriveable in that condition. Motability will be able to decide whether the dealership have exhausted all the options or whether they appear unable to find a solution (although you may be able to convince them that they don’t seem to be getting anywhere). Ask up front just when you can reject the car through a failure that’s not being repaired and has rendered it useless.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark