Ok so I have this solved, and wanted to post back the solution. I wanted to wait a few weeks to make sure this had actually gone for good.
So just to address the previous comment, the plugs had nothing to do with the solution.
I went to take the XC90 on a weekend away, and once I got to 110kmh, the vehicle started vibrating as though the road was corrugated. It then entered an aggressive limp home mode, which was actually more of a limp-to-the-side-of-the-road mode.
I logged into the ODBII and cleared the code P0300 random misfire, and was able to drive the vehicle again, but once I got to 110kmh, it would once again enter the aggressive self-protect mode. If I stayed below 90kmh it was ok, so I limped it to the mechanic shop and left it there.
After an more thorough investigation, they discovered that the PCV and flame trap were all blocked up. I should mention I've had this thing serviced religiously since I bought it at 100k kms, but apparently it can just happen over time? They cleaned out the parts that were re-usable and replaced the ones that needed replacing. Two of the more interesting symptoms were that we could hear a whistle from the engine even though the turbo had just been replaced, which totally went away once the PCV/flametrap work was completed. In fact the engine overall is considerably more quiet now. Also when we opened the oil cap, a light smoke would come out, which is now gone.
Bit of a heartache but now it's running like a dream after the turbo replacement and all this work. Very quiet and plenty of pull under boost, no more oil in the induction system, no check engine lights, no smoke from the exhaust. Now hopefully she runs reliably for a while.
Hope this helps someone if they have this issue in the future.