Originally Posted by ApeironTsuka
If you've previously used adb and given shell Superuser access, you could connect your device to that computer, run this command and then turn the device on.
I hadn't given adb SU access but decided to try that anyway. Doesn't get far enough in the boot for adb to connect, unfortunately. It pops up a quick "Please wait..", crashes back to bootloader, and boots to a "Your system may be corrupt. If this keeps happening yada yada" screen with a try again/factory reset. Since the bootloader on-board is the stock one until this is resolved, it then boots fine from there.
What happens if you flash the new stock (unrooted) kernel to both slots? This almost sounds (guessing since I haven't seen it) that it's failing to boot on one slot then recovering by booting from the other slot. Saying this because you say it fails, then succeeds.
Bootloader will always be stock. Nothing you've done, or attempted to do, effects the bootloader (other than applying the OTA).
Did you patch the new, updated, stock kernel with Magisk or are you trying to use an older kernel?
If he's running stock then he shouldn't be turning dm_verity off anyway.