I think there was another factor at play here which was fear of being found out. It is purely supposition but I think they were flying on autopilot up until the moment that ATC asks them for notification if unable to remain clear of cloud. I think at that point the autopilot is switched off to manoeuvre around cloud and the loss off control occurs. If they had disregarded ATC and continued on autopilot, assuming that Ibbotson had enough knowledge to use the boots and anti ice, they may well have survived.
I said this at the beginning, anyone involved in the procurement of Ibbotson for this flight, knowing that it would depart after dark was acting in a way that is so reckless that they must surely face a charge of manslaughter. There was much debate in the beginning as to whether Ibbotson knew that he was expected to fly the return in the dark or only found out on the day. There was clearly communication between the parties occurring on the day, it will be interesting when we discover if the forces of law and order have any records.