Right well I'm happy to report a resolution to this thread even if it has taken a few months.
DB@xc90 was extremely kind and gave me his turbo actuator to try on my XC90. Things went well with it for a day, with the car jumping off the line like a sprightly young thing again. But then I went and put my foot down all the way - just to give it a thorough test - and, well, the actuator jammed in the fully up position and the car went into limp mode and wouldn't be coaxed out of it. And unfortunately the position of the level-pulling arm meant that I couldn't get the actuator out after disconnecting and unbolting it. In the end my tapping it with a steel rod and hammer to try to get it to move a bit caused permanent damage. So I put the old one back in and learnt to live with it for a while.
What I realised eventually was that the original unit was working fine unless I drove the car enough to get the engine hot, and then parked it for about half an hour, then tried to drive again. At the first sign of the tacho going past 2000rpm it would trip into limp mode again. BUT if I drove it super gently for about 5km I could then drive it normally again. Basically the heat from the engine block in the parked car was heating up the actuator to a point where it would stop working; driving it several km got enough air flow happening (on a Canberra winter's day) to cool the actuator enough that it would then operate properly again. I spent a lot of time pulling over to clear the errors - occasionally it would be "Turbo pressure too high" but almost always it was "Turbo pressure too low." The other errors I'd been having in the recent past were Oxygen sensor errors and MAF sensor errors. As described above I'd replaced the oxygen sensor in the exhaust manifold, but I didn't replace the MAF sensor. I did replace the air filter (long overdue) and this reduced the frequency of issues somewhat, as had the fuel filter replacement. But the other thing I found (with the help of a friend - thanks Greg!) was a crack in the charge air pipe (the plastic one that runs under the engine) starting at the hole for the little screw that helps hold the hose clamp in place at the junction of the resonator and charge air pipes. Patched this up with gasket silicone from the inside and a block of black rubber and an extra hose clamp around the outside to keep the silicone from being blown out. After this the error messages became much more consistent.
Although I felt pretty confident it was the actuator that was my problem I talked to a very helpful person at Turbochargers Plus who told me I had to make sure it wasn't a wiring fault before he'd sell me an actuator. He suggested an auto electrician, but I came up with another test after inspecting the three wires and thinking everything looked like it should be fine. With the engine hot and the car in limp mode due to the actuator being jammed, I parked it at home and quickly pulled out the actuator and put it in the fridge for 20 minutes. I ran the engine for about 5 minutes of this time just to make sure everything else stayed hot, ie: the wiring. Had been driving it around with the cross-brace removed for a while now - and couldn't really notice any difference, but my other three engine mounts are only about a year old so yours might behave differently - and this makes removing the actuator quick and easy as long as you've got a pair of nice fine needle-nose pliers (like these https://www.jaycar.com.au/precision-long-nose-pliers-110mm/p/TH2334) to pull the circlip, and a 10mm socket on a small ratchet for the three bolts.
After the 20 minutes in the fridge I reconnected the actuator and tested it with the scan tool - working like a charm again. Put it back on the engine - working like a charm again. Drove it, parked it up for half an hour, drove it again - jamming like a bastard again. At this point I felt pretty confident the temperature sensitive fault was with the actuator itself. After a wasted week buying the wrong actuator from a deceptive ebay listing I got in touch with PEC Australia (also www.peconline.com.au) via their ebay listings and after supplying them with the codes G35 and 6NW 009 228 off my original Hella actuator, plus photos showing the actuation arm type and position (there are different lengths and orientations for different vehicles) these good folks sent me an actuator that fitted and drove pretty well from the start. I think they had to custom program one to suit but it only took about a week to receive it.
I've driven the car for about a week around town, where it gave no errors and was good, if a bit slow to build revs below about 1500rpm (especially when cold) and quite peaky with the power delivery surging in from about 1700rpm. Just drove it from Canberra to Bateman's Bay and back again on the weekend - also no errors. And I'm hoping - after having done lots of adapting and resetting of the adaptive settings in the ECU during these few months of sorting stuff out - that the power delivery will become smoother as the car re-learns my driving style.
In any case she's back to good enough already, and some definite lessons learnt. I've put the cross brace back in and the undershield back on. Fingers crossed for a bit of untroubled motoring for a while now as she closes towards 400,000km.
Thanks again DB@xc90 for your generosity helping me along the way. I owe you one!
But also thanks to all the contributors on this site and the others that make car ownership more affordable and interesting for those of us willing to have a go.
Pictures from the path...
Inside DB@xc90's not-quite-right actuator (a G26 I think).
The 6NW number is important (RHS)...
...as is the G number (centre, on the side of the allow base, if you have good eyes for silver on silver).
The original (2007 XC90 D5) at top vs the wrong one below that I got from ebay - note the fairly different arm lengths and orientations. This wrong one wouldn't even go through the hole in the mounting bracket above the turbo due to the longer arm, and the orientation of the arm was never going to work! Thankfully I got a hassle-free refund from the seller after going through the ebay refund motions.