If you have a valuable Grade A asset why not employ a Grade A operator to fly that asset around! The cost of an exec jet with experienced ATPLs flying it is peanuts compared with the value of someone like Sala. They would not have flown at 5,000 ft over the sea, VMC, at night but at FLs in controlled airspace. The radar plot of the final stage of the flight appears to indicate a classic spatial disorientation profile, a known problem in the prevailing conditions especially if a pilot is not instrument rated. AAIB will no doubt consider this possibility.
I am not up to date on the shared costs aspect but I believe that a pilot must be considered to be flying on his US licence for this type of sector, in which case the costs would have to be halved with two POB? As has been said the AAIB is not particularly interested in this apart from determining whether or not it is a commercial or private flight and what effect this had on safety,