Well that was a special level of hell... But, just as OP stated, using opodecrypt worked. For those with a Metro N100, you can just use this to flash to the global firmware. It is already patched:
https://mega.nz/file/8fhxkQoR#Yt1ZFJMXsY-wuQIhQ7ar8M_E6PN6wB9NixdPYtWzCQA
If you are using a T-mobile N100, you will still need to follow the guide here if it is still locked (T-mobile unlocked devices if requested at the time): The AT&T N100 (very hard to find) came unlocked and is the global model.
https://xdaforums.com/t/flash-global-rom-to-tmobile-metro-w-locked-bootloader.4387517/
You do not need to contact OnePlus for an unlock pin. Converting to the global firmware will allow you to unlock the device. You do have to use the patched msmDownloadtool to do it, or patch it yourself as described above
Once you have it flashed to global, unlock the bootloader, then on reboot enable developer options and use the UT installer. I set the options to repartition as OP did, and it failed. Retried without the partitions option and it worked - with the caveat that I had to run the installer multiple times because the connection to the server kept dropping. That might not happen to you. Also, the UT installer will error out with a message about setting slot A or B, and an error about the filesystem - hit ignore both times and it will still flash the device. Once again, it may take multiple tries to achieve success. But, when it boots, everything works, except VoLTE which I am troubleshooting.
Also, it does not work with a Verizon sim out of the box, but does with T-mobile.