[2013GV]

It should be sealed no power applied. ECM applies a pwm signal to it to vary its opening to control boost
P0234 is the genetic "open or short circuit " detected.
While the solenoid seems suspect from a sealing point of view, you may also have a wiring problem. Id replace the solenoid as its not functioning correctly and should hold a vacuum when unplugged.

 

[2013GV]

I haven't measured one in ages but a pwm signal will show a varying voltage on a multimeter, only way to really look at it is via a scope

 

[2013GV]

The f9q usually runs around the 15 psi (1.1 bar) but can hit 20 when being worked under load at high revs. Depending on conditions you will probably find it hovering around 15 to 18 psi under acceleration

If its detected an issue the ecm limps about 6 psi and 2800 rpm by dialling back the injection and timing, and opening the waste gate solenoid if required (if boost vanes get stuck open) and closing the boost control solenoid (dropping volts) so your 7 and 3000 sounds about right for a limp.
The boost control solenoid works reverse to how people think, more voltage = more vacuum = bigger turbo vane opening = more boost.

You have done well getting results from a multimeter, looks like your one isn't too fussy about its dc input being chopped

 

[2013GV]

I think you will find its a dicky solenoid giving inconsistent flow thats triggering the code. Ecm is commanding a certain vane opening and expecting to see corresponding boost and air flow and ain't getting it.

Multimeter is not going yo be accurate but at Keats using the sane instrument will give you a base line, but I'm not sure its going to show much difference

 

[2013GV]

Going from my bad memory, I think there's a maf, map and a boost pressure sensor.

Suzuki took a working system developed by Renault and wrote their own ecm firmware, which in my opinion was wrong. They turned a reasonable system into a real pain as the diagnostic codes don't always line up. The Renault forums have a lot more knowledge on these things as they are common and dont seem to give the same troubles.
Suzuki took a transverse engine, mounted it longitudinal and never thought about servicing. Try taking the turbo off, or getting to the thermostat with the engine in place. Oil changes are hard enough.

Not sure what's on the vacuum line, sorry.

 

[2013GV]

In a diesel it doesn't do the same things you expect from a petrol. It will be changing back to its default settings and triggering the limp light, but you probably won't notice an idle change, but will notice lack of power when driving.
A petrol being a bit more sensitive to mixture changes will usually drop or become rough in idle.