Did it for me from the CLI, I'm literally updating to Proxmox Kernel as we speak, I'll check the file after reboot...fingers crossed...
It may not have actually added the repo... if you look in the output I put in post #16 when it runs apt update.. you can see it never hits the omv-extras repository, thus why it doesn't end in an error. My guess is, it didn't actually add the omv-extras repo.
The procedure I detailed above should work though if it didn't.
Edit: Well I see it did work for you.. that's weird.