Regarding the Diff lock lever and solenoid.
The solenoid valve your looking for is mounted on the top of the gearbox at the very back, look straight down inline with the back of the cab. You can see it in your photo of the back of the cab!
There's 2 air pipes on the valve, 1 of them splits into 2 disappearing off down the torque tubes.
The other heads back to a valve of sorts, I believe it only lets air through at a set pressure. Possibly more than your 4 bar..
I'd be checking the lever is all the way up, has it made the 3rd indent looking underneath the cab?
From my own experience the rubber boot on the solenoid goes extremely hard, stopping the plunger operating.
Take a measurement across the plunger before and after you've engaged the hand lever, has it moved?