But you still have the car 18 months after you bought it, you seem to have taken advice from many sources and you are now on a web forum asking for more advice. With respect you have taken far too long, you have been using the car for the 18 months and I am sure added many miles. I am not seeking to argue with you, I am saying had I been in your situation the car would have been back with Citroen well over a year ago. It seems to me you have gone about this terribly and I am trying to tell you no matter what you have been told, you are likely to struggle. I hope you get the result you want, but really stop messing about. Get it back to the supplying dealer and speak with the DP about resolution as all of this advice has not fixed your problem. You have put up with it far too long, you have not pushed hard enough and have been strung along. Accept that, stop with the emails and get it sorted.
Best advice I can offer.
Who is the dealer by the way?
And how exactly was he supposed to have gone about this? He followed the only process they offer. The selling dealership did not need to take possession of the vehicle since the issues were rectifiable under warranty and therefore it makes sense Citroen would have allowed any of their dealerships to resolve the issue. The fact that so much time has passed while the problem persists is the fault of Citroen not him.