Given that the pilot responded to ATC calls immediately prior to the crash, the claimed potentially fatal levels of carbon monoxide found in the blood may not be quite what they appear from a simple reading of the analytical report. The pilot being considerably less fit and much older than the passenger should have succumbed much more readily to the effects of carbon monoxide poisoning. Biological processes are capable of producing high levels of alcohol and other substances in the blood after death.
There can be absolutely no doubt that carbon monoxide poisoning may have been a contributory factor in this incident, but the primary factors are still weather, an unqualified pilot, and an aged aircraft that had apparently not been maintained to the highest standards. Everything was being done on the cheap in order to save money. There's your cause!