Pilot considered himself fit for flight.
You obviously haven't watched the video about the guy who woke up in his crashed Mooney in a paddock in the dark.
The effects of CO are not linear and include impaired decision-making. Non-expert PAX wouldn't notice subtle behavioral telltales of a pilot's deteriorating competence.
- CO detector showed nil indication.
What kind of CO detector was it, and what was its 'use by' date? Again, if you watch the video, comments are made about the reliability of traditional 'card' type detectors, even assuming they haven't been glued to an instrument panel forever. Plenty of other data about the reliability of them.
From the above, I think we can reasonably assume CO was not an issue prior to takeoff of fatal flight.
Speak for yourself.
And I note I'm no fan of ATSB, given the frequency with which it produces works of fiction and unhelpful mush.