I've been away on holiday a few days so I'm late to see this thread but I've done plenty fuel system related work on Supercharged Rangerovers (I've converted plenty of them to LPG).
They have a returnless fuel system but fuel pressure is adjusted by electronics controlling the fuel pump... It could be termed 'on demand' fuel pressure. The engine management on them is very sensitive to vacuum leaks (including false air from a disconnected or failing cam cover breather, evap purge valve flowing when it should be closed, etc).
One unusual aspect of them - Say you've had an error code, then you reset the error code (which after some error codes will reset fuel trims)... If you then just start (after resetting fuel trims) and leave it idling the chances are you'll get fuel trim error codes. The way around that would be to clear error codes then not leave the car idling, instead start it up and immediately go for a drive.
With a basic scan tool you probably won't be able to manually reset fuel trims. But causing one of the more 'severe' error codes such as open circuit petrol injector then resetting error codes will reset fuel trims. I.e. If you want to reset fuel trims but don't have a scan tool with that function you can still reset fuel trims by disconnecting a fuel injector, let the system log the error code, then clearing the error code will reset fuel trims. But then don't forget to go for a drive immediately after starting the engine or you'll probably get a fuel trim error code anyway ;-)
A scan tool will show you fuel pressure... There's an electronic fuel pressure sensor built onto the fuel rail, there has to be one for the on-demand fuel pressure system to be able to read fuel pressure and control the fuel pump properly. Edit - Noticed you're getting 40>60psi, that seems about right to me, as memory serves my scan tools read between around 400 and 600kpa on most Supercharged Rangerovers depending on engine load.
The lambda sensors are wide band, any voltage reading from them is meaningless (a voltage reading from a wide band probe is meaningless without knowing how the 'pump' in the wide band probe is being controlled) but an OBD scan tool should be ale to tell you directly what mixture is, probably in terms of lambda (lambda of 1 means correct mixture, 0.995 slightly rich, 1.005 slightly lean). Mixture should stay close to lambda of 1 (which is 14.7:1) but go rich under boost conditions maybe down to around 0.8 at full engine output. The scan tool can tell you mixture because the engine ECU knows mixture, it contains a dedicated wide band lambda sensor control chip that 'homes in' on mixture by reading lambda voltage and knowing how much current (uA) it is applying to the 'pump' in the wide band probe.