For what its worth I think the most likely scenario based on what the report reveals is that the long history ( as recorded in the dodgy dossier of unapproved e mails) of autopilot function and indication issues is what did for the guys.
Took off normally ....engaged autopilot at first opportunity as IMC. It didnt engage in vertical mode...and too late Rob realised it was not following flight director.
I make no aplogy for speculating as the report is so inconclusive........