Maybe write 2 letters. 1 says what you really feel. Do not post it.
the second is the humble one, but with as amany hard facts as you can assemble.
Once you have a case handler, be as nice as you possibly can to her/him. they are your way in.
A p.s. about "different racks at the exchange". When my BB with BT was giving me grief, I think I was put on 4 different racks. (Or was told that I had been). None of them made the tiniest bit of difference).