Following my questions re phone soft-bricked on trying to calibrate proximity sensor: many thanks for responses. I've tried installing MR_Bump after various other actions, but the fault always remains exactly the same, an error message on attempting to boot "[670] boot certification verify ...", followed by alternating red and blue flashing light.
As the phone has other problems - proximity sensor keeping screen off after a call, whether hardware- or software-related, and dodgy USB port, I had decided to buy a replacement, a phone sold as a new, boxed D855; but I was sent a D855K (Australia), so I'm back trying to get my existing phone working, at least for now.
As I said, it's a 16GB LG G3 D855, rooted, with TWRP installed, running fulmics 6.9 Marshmallow (I bought it like that).
My latest attempts were to install LineageOS 15.1 or fulmics 8.0, followed by MR_Bump (clearing caches before and after). I don't know if the repeated flashing of different things has made matters worse. I have also updated TWRP to the latest version (I hadn't realised this was possible, given an older version installed). Do I need to unroot the phone first, or install a stock ROM?
I have collected a number of files; could anyone suggest which combinations I should try (or do I need to download something else)? All files are for LG G3 D855 where relevant. I can't make a USB connection, probably because the USB port is not working properly. I have wiped everything except SD card and OTG, many times.
lineage-15.1-20190724-nightly-d855-signed.zip
FulmicsROM-8.0.zip
recovery.img (has been on SD card since 2017)
twrp-3.0.2-0-d855.img
twrp-3.3.1-0-d855.img and twrp-3.3.1-0-d855.img.asc
MR_Bump.zip
D855-MM-prod-bld2.zip
LG-D85530B-Flashable.System.zip
SuperSU-v2.82-201705271822.zip
LG-D85530B-Flashable.System.zip.part
LG-D85530B-Flashable.Modem.RPM.TZ-AutoHybrid.zip
BETA-SuperSU-v2.52.zip
LG-D85530B-Flashable.Boot-AutoV2.zip
A TWRP nandroid backup made in 2017
A TWRP nandroid backup made after the bricking
Best wishes