MDP, I so agree with the sentiment of what you are saying - once you are PIC, the buck stops there. If you make a series of errors, it's your fault, not anyone else's. In fact, in today's litigeous world, it does irritate me to read of people trying to fix the liability for their mistakes upon innocent third parties.
And yet, and yet, there is more to this particular tale than that, as there is another dimension which does need more discussion and investigation; according to the aaib report, the young pilot concerned was contacted by the CFI who offered him the opportunity to act as PIC on this flight flight, in order to give the pilots of the twin a lift down to Exeter, with the benefit of a more experienced pilot sitting P2. He would pay only the price of the fuel to so do.
That CFI must have had chapter and verse on the W&B limitations of a PA28 140; anyone who has owned one knows that 4 adults plus fuel - even to tabs (128l therefore 3hrs flying time, plus diversion) - will result in an aircraft which is out of W&B and CofG by quite some margin. You don't need to work the detailed calculations - you know that it won't work - period.
Whatever the reasons, that flight should not have taken off for three reasons that were well known to the CFI;
1. W&B
2. Currency
3. The aircraft was not equipped for the prevailing IMC conditions
I can't think of a flying club that I have flown from where a renter would have been allowed to depart under these circumstances, so I'd love to hear what the coroners court has to say as why this flight, this club and this CFI was so different.