So, where was he -- in the lavatory?
Or... wasn't the FO's wife on the flight - would it be polite to say hello and chat whilst hubby is PIC?
For me the PNF left seat telling PF to stop climbing several times was the probably the most puzzling data point.
If the guy next to me is repeatedly non-responsive and executing a dangerous change in flight level then it ones duty "I have control" and press priority button, and I expect some confirmation in response "roger, you have control" or whatever the French equivalent is.
It was clearly time to put their game face on and switch from sloppy banter chat to professional communications.
Who knows maybe they were impaired by fumes/gas - this is not a judgement just an observation.