They can be registered to two hosts at once, I tried it last week and it works, but it doesn't make sense to me to do this for other reasons as I explained before.
Other than that the advice you have been given sounds much as I explained before too.
I would add that not needing this in the last 7 years may have been lucky as I suspect that you might not have known to delete the lock file in order to bring the VM up on the spare server.
No, you're right, I wouldn't have known to delete any lock files to bring the VM up on the backup server. Thankfully that company does our backup so they are my backup in case something were to happen. Going forward I have learned more about our network and the setup that we weren't privy to in the past. This time I have done most of the setup and configurations in preparation for the upgrade. This was the most important thing left that I didn't understand. I am going to watch like a hawk to see what he does. Not that it matters, he usually explains to me what he is doing now that he is the boss. LOL
I'd echo again to your bosses that vCenter would be a good idea, and with vSphere Essentials surely not something that would break the bank.
Good luck with it anyway.