Thanks to cidxtc. If I understand correctly I needed to find and flash a new kernel for Marshmallow. The Xceed kernel seems compatible with fulmics (file D855-MM-prod-bld2.zip), so I tried, wiping everything except SD card and USB OTG before every attempt and wiping both caches afterwards (overkill, I'm sure):
- Restore from 2017 nandroid backup and flash kernel
- Restore from post-crash backup and flash kernel
I couldn't install fulmics 8.0 because of the twerp 3.1+ issue; I tried extracting the boot.img (LZ4 version) and installing it first (as I mentioned in my original report) first, but it still didn't work. I was very careful only to use D855 files throughout.
To make matters worse, I think the USB connector on the phone is faulty; I have to wiggle it sometimes to get it to charge, so the signal connectors may be duff; it isn't recognised by a PC with Qualcomm drivers installed.
I see there's a suggestion regarding Mr Bump that I might try, but maybe it's time to give up, with soft-bricked phone that has proximity sensor problem (unless that was a software conflict) and wonky USB connector. And my failed attempts may have made matters worse (though the symptom is always the same). I'll report back if I do decide to give it another try. Of course, my total lack of Android and even up-to-date Linux knowledge doesn't help!
Thanks and best wishes
The mr bump is exactly what I was think of. That is what you need to flash to get everything working.