I am sorry you are having problems that have not been resolved. I don't know how far everything has been looked at from your setup/BT/Merula routing, but from what you say Merula have certainly put in some effort to find the problem.. Having said that it not acceptable for them not call back when promised, unless you missed a call.
All I can say that I have been a home Merula customer for approaching 10 years and they have always strived to sort problems out. There have been a very few times where this has taken longer than I would have wanted, but they have always sorted things in the end. Generally any problems have been sorted immediately and the reason I stay with them is because of the excellent service i've had.
I would email support again with your concerns and say you want an answer ASAP or else!
IME, assuming proven micro filters and different routers this sort of problem is a line problem despite the BT checks. AFAIK, Merula are not limited on capacity. I have been involved with several similar caes and despite NFF reports, the fact a BT bloke has remade connections has often solved the poblem for good.
I do hope you get things sorted, it is so [censored] frustrating. I am sure Merula will respond if you try again.