Alistair, really sorry to read of your current predicament. However I think you were possibly prepared for some aspect of that which you are currently experiencing... It does seem that the more we have to reply on corporate systems, the more potential there seems to be for a foul up along the way..?
I wonder if it might seem reasonable to take some measure of control of the process yourself..? Perhaps by ordering the screen from the MMC to be delivered to you then arrange to have it fitted by whoever you think might be capable of carrying out the task..
I think I typed recently of my 991`s windscreen replacement... If you have read of the other posts on the variation of quality of screens, I was in no mind to have my screen replaced with either a cheaper or substandard item, or to have it fitted in any way which was not up to original spec.. For sure I was looking for the cost or part thereof to be covered by my insurance company but, I was prepared to pay for the task fully myself rather than opt for a sub standard alternative.. To that end I had my Porsche dealership order the screen and have it fitted, my expectation being that they would likely employ Autoglass or others to carry out the actual fitting process, which bothered me not, as given I tasked Porsche with replacing the screen, it was down to them to provide a quality of service matching their brand marketing.... Bottom line if anything had been damaged during the fitting process such as scratched paint or damaged trim etc.etc.etc., then they would have more clout with the fitter than would I...?
Fortunately for me all of that worked out better than hoped, in there was no issue relative to fit and finish, and Footman James who had initially suggested there would be a limit placed on their windscreen replacement, paid for the task in full.
Whatever.... Again so sorry to read off what is undoubtedly a very frustrating situation... Just wanted to providee an alternative line of thought..
Hope you get it resolved soonest Alistair..