[G4 Q&A, Help & Troubleshooting] verbage:
So, straight to the point--anybody here have an idea about how to relock the bootloader on an H811 when ONLY download mode is available?

Over the last few days, this H811 has progressively succumbed to the dreaded G4 bootloop plague, and it is now purely bootlooping (doesn't even get to T-Mobile logo). Before it got so bad, I was able to flash it back to factory state (minus the unlocked bootloader) with the 20x KDZ using LGUP (reason why I did this will be explained below...). But this means that recovery is not available to get get into bootloader mode that way. And I have not yet had a complete successful boot after the KDZ flash to enable USB debugging, which would allow me to get into bootloader mode with the adb reboot bootloader command. So essentially, only download mode is available at this point, uggh...

The back story is that I just bought this used H811 to try out steadfasterX's LOS16, which, by the way, is fantastic! And almost immediately, the phone started displaying bootloop-ish symptoms. For example, like after the initial bootloader unlock, which I did immediately on receiving the device just a few days ago, the phone crashed during the optimizing apps step of the subsequent reboot (it probably overheated). I should have just relocked the bootloader at that point, and sent it back to the vendor for replacement. But I was anxious to try LOS16, and between occasional crashes, I eventually got it installed. But over the next few days, the crashing and occasional bootlooping became so progressively worse that I decided I really did need to send it back for replacement. So today I reflashed the 20x KDZ with LGUP, and that worked fine. But I'd like to relock the bootloader, too, so the vendor doesn't claim I screwed with it, and thus, that they are not responsible for replacing it. But as I mentioned, the only mode available at this point is download mode because it is purely bootlooping on the initial text screen without even getting to the TMo logo.

If I could get into bootloader mode, I could try to do either a a.) fastboot oem lock (given it is fully stock with the recent 20x KDZ flash, there should be no chance of bricking...), or b.) I could also flash just the misc partition extracted from the 20x KDZ. But again, without recovery, nor the possibility to enable USB debugging, I cannot get into bootloader mode. So that's why download mode is my only option.

steadfasterX's SALT works with download mode, but it cannot flash single partitions yet to follow the misc partition flashing strategy I mentioned above.

So what options does this leave me?

Option 1.) phone is in the freezer right now to see if I can get just one more good boot in order to get enable USB debugging, and thus, get to bootloader mode via adb; and from there, then relock the bootloader so the vendor doesn't see the bootloader unlocked message, and claim that I am responsible for the problems...; yeah, the freezer method is a long shot, but in the past with a VS986 that was bootlooping, I did get one final boot out of it for data rescue using this freezer method...

Option 2.) in download mode and via SALT, I can get a shell into the phone; maybe there is a way to flash just the misc partition (extracted from the 20x KDZ) via a command line in that shell, and thus, get the system back to a locked state?

Option 3.) well, send it back to the vendor as is, and hope that they do not try to claim it is my fault because the see the unlocked bootloader message...

Option 4.) if options 1-3 do not work, well, perhaps give LG a call to see if they will fix the circuit board; by the way, they did it for the VS986 I mentioned above without any questions whatsoever; but the difference is that I was the original purchaser of that VS986 whereas this H811 is a secondhand purchase, and LG might use that as a reason for not fixing it.

Any other options that I might be missing? Thanks for any wisdom or suggestions you might be able to provide!

Anyway, my learning lesson from all of this is patience--like I said, I should have returned the device immediately on seeing the first signs of bootlooping because in my experience with both the VS986, and this newly received H811, well, once bootlooping symptoms start, they progress very quickly...