After posting the above message I decided to just give it a try, though it felt like trying to throw a dart in the bull’s eye in pitch dark.
So I just enabled the OpenXR plugins again and tried to package the default VR template. I did get a build error because I had made a silly mistake in configuring the Android sdk and ndk settings, but when I fixed that it built. Just like that.
I haven’t yet deployed it on the PICO but will try that later this weekend when I have some time. If it all works out I’ll note down everything I did, maybe it helps someone else 
PS though I answered the last of my questions above I’d still be interested in answers to the others 