Weight on his mind
Not really sure how anyone could assess this flight to be anything other than 'commercial'. McKay was an agent in a significant commercial transaction where an asset under his control was purchased for £15m. Released emails from McKay to Sala affirm that his relationship was strictly professional- in fact he openly stated that there would be no personal relationship, in his words "just want to make money". As part of the commercial transaction, the asset had to be delivered to it's new facilities. His new employers offered a commercial flight. McKay arranged this alternative transportation. McKay has since said that he 'wished he'd told Sala he couldn't go that weekend'- therefore McKay has admitted to having sufficient control and authority over this asset. McKay had the responsibility of assuring the safe delivery of his asset. McKay did not choose an appropriate commercial operator- he instead selected a cheap alternative with no AOC.
The cheap alternative, working in a very casual and informal way then sub-contracted an even less adequate and less qualified individual. All of which is a bit smoke and mirrors. Dont try to pull the "he wasn't paid card". That's the way they've been trying to portray this since it happened. Dave Ibbotson was made to go out of his way to carry out this delivery. I've seen wingly being discussed as being marginal... but with wingly... generally the pilot is going to a specific location on a specific day anyway and is offering a lift for someone to help with fuel & landing costs etc. This is much different- Dave Ibbotson was sent to Nantes at short notice- significantly out of his way- to bring an asset back. Sala's delivery to Cardiff was due to, and a part of, the larger commercial transaction. Dave Henderson, if he has a trusted and longstanding arrangement with McKay will know that he would be fully compensated for his services- be it cash or other. Ibbotson I'm sure had the same expectation of payment for his services- whether the specific amount had been prior agreed or not. Even if he "owed a favour", it's still a commercial service, just as it would be if Ibbotson DJ'd for Henderson's wedding or serviced his boiler. Just because it may not be invoiced at arms length doesn't preclude it from being a commercial relationship. I'd say if the boiler leaked or caused an explosion or death, Ibbotson would have been brought to book and if at fault possibly prosecuted and lost his accreditation regardless of whether it was 'just a favour' or not.
In legal terms if I were Cardiff/Nantes- I'd be chasing McKay for the £15m. As far as the flight arrangements go, I'd say Henderson has a lot to answer for, in essence operating a commercial service without the appropriate licences and all the checks and balances that go with being a fully certified Air Operator. Again, this young lad was the asset, worth about 5 times the value of the plane when brand new. The only reason for his journey was for commercial purposes. There was no personal connection.
In terms of investigation, I'd imagine it crucial to recover the aircraft and as much evidence as they can find. Ibbotson's phone would be invaluable for the investigations. For example to see text messages and phone calls with Henderson and others during arrangements and on the day itself. Possible attempts to contact during the flight? At under 5,000 ft, signal would be available. We also know the unfortunate situation in which he was placed, he would have been very reluctant to relay problems to ATC fearing it may open up a can of worms, but did he attempt to contact others via his phone when in the air? Phone may also give info from skydemon or other apps.
Also, in terms of investigation, surely one of their initial tests would be the PIC's toxicology? Or even a post mortem to check if there was a sudden medical emergency that incapacitated the PIC? I would suggest that the AAIB investigation team would be very keen to recover the wreckage.
Another factor that I'm not sure has been mentioned, but would anyone think weight and CG might be an issue here? I've read that there were three failed take-off attempts but haven't seen any verification. Remember Sala was moving home, left his dog in kennels, how much did he bring with him for his move- was it everything and the kitchen sink? Significant weight, CG too far aft, with probable icing in IMC at night under the control of a PIC whose PPL stated that he was restricted to only VFR during the day?