remember the Paine Stewart Lear 35 accident
Or the Kingair that depressurized Perth to Leonora in 2000. There's been more than a few depressurized events in aircraft like this. I'm in no way saying that this is the only option, it just seems to fit so far in what little information we have.
I would revisit pressurization if the aircraft had long since passed it's descent point and there was no radio transmission.
It was 80-90 NM east of Isa when it departed FL 280. That would put it on profile for a 3x if it was intending to land at Mt Isa, however it did not seem on track for Isa at the time, I would assume that descent clearance/traffic should have been passed on by then, so it will be interesting if they had responded to that.
I wouldn’t have thought that it would have a complex pressurization system, so I’d expect that even if they didn’t set the pressurization to go back up to F280, all that’s going to happen is it’s going to hit max diff and open the safety valves to maintain max diff. The cabin isn’t going to magically depressurize because it wasn’t set to climb back up to F280.
I'm not familiar with the pressurization system on this aircraft, but one thing I do know is that there is many different systems that work in different ways. Some you just set destination elevation and it's set and forget, others you have to be more interactive with what is set and intended cruise altitudes. My angle was how does this system work, does it automatically enter descent mode and continue descent (depressurize) unless told otherwise or is it more automatic and can sense intermediate level offs and climbs, or is it more manual with a need to set cruise altitudes and set schedules, the later seems unlikely as these are post 1980s aircraft, but stranger things have been built. I'd also be interested in the fuel system, does it have aux and mains, or just a set of interconnected mains that feed tank to engine. All I can find is that the 690 had 3 tanks with a center tank, but the 695A streamlined it somehow and reduced refueling times.
I'm also still struggling to see a structural failure element in this. If a wing separated or deformed I can't see how the autopilot managed to keep it straight and level until it basically stalled, if the pilot was flying why did they not initiate a descent to keep speed up to maintain control. None of this fits with a structural failure, more a loss of power that led to speed loss and departure from controlled flight when it reached near the stall.