I only have the information in the article, but from that it seems like the investigator originally assigned to the case was lazy, incompetent, or perhaps biased against this family because of their history with the police. That last thing seems like it is (or should be) illegal but the first two aren’t, and the investigator is retired so it’s too late for him to be penalized by the department (although I suspect that they wouldn’t have). Other than that, the behavior of the police appears to have been appropriate - any driver could have hit a man crossing a six-lane highway at night, it isn’t the coroner’s job to track down a dead person’s next of kin, and the new investigator assigned to the case appears to have been diligent.
IMO the behavior of the first investigator justifies an internal investigation and I would be quite cynical about the impartiality of that investigation, but I don’t see strong evidence that a crime took place.