If (big "IF") the crack in the exhaust that ATSB says was there prior to the accident, was in fact there prior to the accident, the potential for CO respiration would seem as plausible as a passenger knocking the pilot out or Angel Flight-related pressure. But if (big "IF") a crack of that size was, in fact, in the exhaust prior to the accident, what maintenance inspections were done and certified by whom in the previous year or so?
As with the Essendon tragedy, it will probably be the legal action commenced by the NOK etc of the deceased that will be the only effective avenue for the discovery and exposure of the truth.