After many hours of diligent back and forth last night, I gave in and restored from a backup, my last stop before flashing back to stock. That actually worked. So I'm back to the point of being rooted with TWRP installed, (Seriously, make a backup) but at least I have still have all my data.
I've actually used the stock LG interface for more time now than I did after I picked up the phone. God it's awful. I can understand that as an OEM, you need sales bullet points, and something to fill a product launch with, as such you largely have to add to what exists in stock android. But I doubt they ever did any UX tests, either that, or the focus group they had were all "normals" who've never seen a smartphone before. But I digress..
I spent most of the time getting adb to work again, and experimenting with that. I re-installed the drivers, available at my first link above. After that nothing seemed to work, so I manually found the 64bit USB installer and ran that, still no joy, then rebooted the PC. Once I reconnected the phone, then the installers kicked in, though MPT failed. But all you really need is the ADB interface.
I also found out that "adb devices" only ever sees the phone when it's booted into the system, and stuck at the boot animation after the LG logo. If I booted into TWRP recovery it, even in sideload mode, Nothing worked.
adb is part of the SDK which you can get from here:
Though all you need is a couple of binaries and a few dll's, and you can find all that in IORoot25 here:
These are basic guides for adb:
(push & pull)
The adb shell is UNIX, not DOS, just FYI. these have you covered:
I used it to upload more PA roms, gapps, etc. None of which worked. regardless of how I did it. Dirty or clean, (with the exception of fully wiping data.) I found the most useful functionality was the command, adb reboot recovery, which will drop you directly into TWRP without the need for the Vulcan handshake. Which is often frustrating, a bit hit & miss, at least for me.
I tried to flash 0429, 0430 & 0501, none of them worked, though I was on 0430 when it borked.