Would the cold start exhaust valve being, stuck closed, cause the ECU to put the vehical in limp mode?
We have the older Bongo... when our 'cold start' exhaust valve jammed, we experienced some loss of power on hills, but the main thing was that the fuel consumption went from 26mpg to about 17mpg.
I've been looking through the forum again to try to find some help for you.
Did you previously mention fault code 1228?
I mentioned before that 'BongoJim' has the newer type Bongo like yours. He also had that fault code.
I've looked back through his posts about it, and he had many tries at fixing it, as have you.
He tried also tried a new sensor and a new turbo, but still had the problem.
Eventually, he discovered that the problem was due to a vac tube collapsing under certain conditions.....therefore restricting vac...
SEE THIS LINK
Also 'slingersurf' has the newer type Bongo and had similar problems, which took a while to sort... he eventually found the solution himself by manually moving the vacuum rod several times (under passenger seat) & spraying some contact oil on electrical plugs.
SEE THIS LINK.
Unfortunately this is all I can find about similar problems on the newer shape Bongo.
If it doesn't help, I'm sure your mechanic will be able to sort it. These things are easier when you can 'see' the problem.

Allans Garage retired. Try PGS (Plymouth Garage Services) or Mayflower Auto Services Plymouth