Why would Cameron give Vince Cable a hard time over it - unless QEII had specifically made it known to her PM that this was her wish, and Cameron was deferentially passing this on to Cable (who luckily wasn’t having any of it).
That is always possible - and if that's the case I hope we find some evidence of it. It would be wrong both of the Queen and of Cameron.
The other sizeable elephant which everyone seems to step around politely is the Mandelson/Epstein aspect.
I simply do not believe that AMW was the only person caught up in their machinations: he wouldn't have been worth it, but he was useful to them while he was there.
A reminder: Mandelson pushed for AMW to be trade envoy. From 2001-2011, Mandelson was Britain's European Commissioner for Trade (2004-8), Business Secretary (2008-9), Secretary of State for Business/Trade/Industry (2009/10). Once Labour lost the election in 2010, he founded the Global Counsel lobbying firm (2010) and became president of the Policy Network (2011), among other activities.