1. I'm not sure I understand the fixation
on what seating arrangements Pilatus sells. The PC-12 is marketed as a very flexible design. Once they leave the factory, the seating can be rearranged.
Strip out all the px seats for a cargo-only configuration. OR, if the plane is only being used for people-moving, a 10th px seat can be bolted in beside the (unneeded) rear cargo door.
In post #13 there is a link to a PC-12 for sale that advertises "Seating: 12".
They exist, regardless of whether they appear in the Pilatus sales brochures.
2. Since no one survived, and there is no CVR, and there was no reason given by the pilot to ATC, obviously the purpose for the diversion will remain a mystery unless it left physical evidence that survived the crash. It could have simply been the need for another potty stop (as in Oroville). It could have been the fuel burn was higher than expected, and the pilot though Bozeman would be stretching it a bit too much (which does not mean that he was oh-my-god out of fuel - just playing it safe).
It could have been something much more serious - but there is no evidence at all one way or the other, yet.
Although - if it WAS serious, there are several airports much closer to the point where the plane turned towards Butte (roughly over Challis/Ellis, Idaho). Friedman Memorial near Ketchum, Idaho, for example.
The diversion point is over the highest mountain ranges in Idaho. An encounter with a mountain wave, given the - interesting - passenger seating, might have caused injuries.
But again, I'd have expected a diversion to the NEAREST airport in that case (50nm or less) rather than to KBTM (100nm).