The AAIB just investigate the cause of the accident, one would assume to report on anything significant that might effect other aircraft's flight safety.
To that end the aircraft should have been pulled from the sea floor and at the least the exhaust tested to confirm or deny the CO2 theory.
It just seems they are hiding something.
It's then up to the CAA to investigate parties concerned as to the legalities of the flight, with the police then going i assume to cps for any prosecutions.
And what of the other pilot, did he not feel a duty of care knowing the experience of the crash pilot.
What was his reason for not flying the aircraft. I would suggest we will never be allowed to know.