Hi DaveYou really do put some effort into your post's the therefore you will always get the answers to your questions.
I would say, the turbo Actuator is working correct and I think your theory is wrong.
Please confirm if your engine has PSG 5 or PSG 16 fitted. Quick check is there one or two Connectors on the ECU thats on top of the Injection pump.
The turbo boost control solenoid controls the movement of the actuator , which in turn moves the waste gate opening or the Variable geometry of the Turbine wheel depending on what type of turbo is fitted.
As a fail safe the turbo needs Vacuum to the actuator to generate BOOST and is mechanically moved to NO BOOST when open to atmospheric pressure.
The ECU controls the Boost control solenoid by Duty Cycle (Pulse width modulation).
Therefore the OCR (Open cyclic Ratio) will be very high at idle 75 to 85%. This allows all the vacuum to act on the diaphragm inside the actuator and move the waste gate lever. The turbo is now set to delver full boost when the engine RPM increases.
Has the turbo boost increases the ecu measures this via the boost pressure sensor as it approaches maximum boost it starts to reduce the OCR the reduce boost. Normally once it reaches to around 10 to 5 % OCR this means the valve is fully open to atmosphere and the turbo is no longer boosting.
So if you connect your mitvac to the actuator and apply full vacuum to the waste gate then the turbo is set to full boost. If you cannot achieve full boost on live data now you have a boost leak , or a boost pressure signal error or a air flow restriction or the new Turbo is faulty.
HTH
Darren................