There are lots of external factors here that with the gift of hindsight need looking at to help prevent a similar situation from an operational perspective.
But it still stands that the accident flight was the responsibility of the crew. They had the option to refuel in Exeter and the option to divert many times on the way back. They chose to push on and died. That part is simple enough to ascertain.
Alluding to who might have been PIC on the way back is a waster of time. Andrew Walker should have clearly established the operating status of the other guy if he believed the flight to be Instructional but it still did not change the fact that between them as crew they had plenty of opportunity to avoid the circumstances that cost them their lives. Or are we now trying to say the the other guy who died was PIC and shift the blame? As far as I see it they were both to blame and paid the ultimate price.