Clutching at straws a bit now, but how about checking the turbo actuator? If the vanes are not moving for some reason (stuck actuator, split hose, whatever) they will stay in the default, open position, which will mean low boost at anything other than full revs. This would hit your mpg.
Doesn't match the original fault though, which was positive charge deviation - but that could well be down to the EGR valve sticking open.
Does your EGR shut properly, were you able to check it when you cleaned it? Might be worth disabling it (unplug the N18 electrical connector) to see if this helps?
Otherwise, perhaps injectors? I think they do wear out, but 120K seems a bit early for that.
The fuel pump on the TDI110 (and the 90) is also the metering device and contains several sensors and actuators, basically that's where the engines power delivery is controlled. You'd need to take it to a diesel specialist to have it diagnosed - but again, it's not a likely place for a failure.
Best of luck, keep us posted.
