He hired Nashville musicians then recorded all but one of his albums in the UK. Crazy decision for me when there are so many great musicians here that could do what he wanted, would be ready at the drop of a hat for recording at BG (not just phoning it in) and that would likely suit his music better than a country Nashville type. I'm not saying they can't play what he wants but they are clearly more at home with the stuff he churns out nowadays.
The 96ers are his core band and don't forget the folkies he added.
Mark would not use them for so long if they could not deliver what he is looking for.
Remember how Mark developped Marbletown live for example.
And with all respect, regarding session musician, I think you can't find better places than Nashville and LA when you need some top hired guns.
I know exactly what you mean and I will acknowledge some of the very best live and work in Nashville, however that doesn't mean it should be the basis for starting a band or that they will be the best for playing the type of music he is going to be playing live. It's like building a football team with 11 top class players but no thought of them being able to form a team or being able to play together.
I think the single reason for the formation of the band Mark ended up playing with is they're all multi-instrumentalists. Everybody plays everything, so it's not a 20-piece orchestra. Richard plays a whole truckload of instruments, Glenn plays every bass imaginable, keyboard players can play the organs, accordions, guitars, synths, percussion and pianos, and so on. Mark went a little over the top with multi-instrumentalists in my opinion, but probably for a reason. His music is pretty diverse, too.
The only non-multi-instrumentalist in Mark's band is probably the drummer. Or, they should sing, too!