[LG V40 ROMs, Kernels, Recoveries, & Other Developm] whitedavidp:
Success!

Thanks again. I tried following your exact steps:

1. Using TWRP, I switch to slot b then powered off. This started some odd bootloop but holding Volume Up and inserting USB brought me to download mode eventually.

2. In download mode, flashed V405QA720e_01_NAO_US_OP_1230.kdz using partition dl, first selecting ALL and then unchecking laf_a, boot_a, and boot_b. This completed successfully but did NOT reboot by itself into TWRP. Instead it booted Android. LGUP did not wait for me to do anything after completing the flash and booting into Android. Grrr.

3. I booted into TWRP as quickly as I could, did nothing at all in Android setup except shut down. Shows I am now in slot A, cannot mount Vendor, backup shows data as zero bytes. So I am guessing I am on my way to encryption hell once again :) At this point there seems no purpose in following the remainder of your protocol since it appears the "damage" has been done by the reboot to Android on slot A.

4. So I got into fastboot and confirmed it thinks the current slot is A using fastboot getvar all. I then used fastboot --set-active=b and this started an immediate reboot into the odd bootloop from which I was again able to enter download mode. I once again flashed the .kdz and was waiting at the time of the reboot to use the hardware buttons to get into TWRP ass soon as the reboot started. That worked! It left LGUP feeling a bit out of sorts but I just shut it down.

In TWRP I then:

1. Accepted system modifications.

2. Check my current slot and it was A. But I'll be darned - I am able to select Vendor for the 1st time ever! At this point and went back to following your protocol.

3. Re-do the RAM disk.

4. Rebooted to Recovery successfully. Went to mount and verified I could still tap vendor and get a checkbox and did so with system as well since you mentioned it. Using the File Manager I can actually see things like an /etc folder in /vendor. Looking good!

5. Did the push of the fstab to /vendor/etc, did format data, did the push of .magisk, and finally tried to install magisk (used 20.4). This died quickly with Error 1 right after displaying "mounting /system, cannot mount /system, unmounting partitions". Hmm. I did see that you had tested mounting both system and vendor and I did the same. Perhaps since system was already mounted, the magisk installed could not mount it again? So I went back and managed to uncheck system. Same result on another magisk install attempt. So went back and unchecked both system AND vendor - this leaves only my external_sd checked - and tried again. No luck.

6. Decided to try rebooting to recovery again and looked around. In mount, only data and my external_sd are checked. Using file Manager, I see the .magisk is still in /data. Tired to install magisk once again and it seemed to work. Hurrah!

7. So now the acid-test: reboot to system and it goes to the Android setup. As soon as I see the Android setup, I shut down and went back into TWRP to see what I can see in /data. I'll be dipped! In backup, data shows 1218MB! File Manager actually shows files! I have never seen this before on this device! It seems that encryption has indeed been defeated!

8. On to the Android setup... Magisk Manager in there and runs. I think that finally - with all of your help and a bit of luck/trial and error - I have things where I need them to be. So to prove the point, on to doing a first real TWRP backup. At this point I am selecting anything with a non-zero MB and excepting the system_image and vendor_image which I am told are redundant to system and vendor. The backup completed successfully and is beautiful to see!

I just cannot believe that I am released from hell thanks to your efforts and those of all who have responded to my many, many newbie posts. I have documented all this in this lengthy post as a record for myself and possibly as a semi-guide to others who may find it useful.

I can finally see the beginning of the end in this transition from my old Samsung Note 3 with Android 4.4.2. Thanks once more!