I need information about credit negotiation....?

The reason that there is no "concrete data to backup success" is that there have not yet been any successes. That might be a slight exaggeration, but not by much. The best "solid information" available is a study done by the government of the State of California, which fund that the companies made no attempts to negotiate at all; they simply collected as much money as they could from the customers and then moved onto their next victims.