I think that the other risk of constantly chasing a Mercedes dealer, especially if they aren’t entirely clear what they are being asked to do that they aren’t doing already, is that they will lose patience and say “I’m afraid that Mercedes no longer supports Motability and we can no longer get the car”.
Personally, if I hadn’t received something concrete that the car had definitely been ordered after a few months, I would have cancelled and started again. The worst scenario would be cancelling a live order after all this time though, especially as there is nothing comparable left on the scheme and you’d be paying more for less.