Enable Auto Unlock with FileVault

I don't have an answer for you, but you are going off into the weeds looking for an incompatibility between FileVault and Auto Unlock. That's not the problem. No Terminal commands are required or desirable. All that can possibly accomplish is to create a mess, so undo whatever it was you did. Perhaps your friend was attempting to "hack" an unsupported Mac into making Auto Unlock work; there is no way for me to know what that might have been.

It appears that you are confusing Automatic Login, which is not available with a FV encrypted startup volume. A login password will be required. That is unrelated to allowing your Apple Watch to unlock your Mac.

Do not overlook any of the bulleted items described in the Apple Support document you read, even if they seem inapplicable or trivial. Also, you may wish to refer to Unlock your Mac with Apple Watch. There are software and hardware requirements, all of which you appear to have met.

In addition to those, research your router. It forms an integral part of the "unlock" requirements. Try restarting it, or research the possibility of a firmware update.

Needless to say, if you are using any non-Apple "anti-virus", "firewall" or similarly categorized "security" junk, anything at all in those broad categories, don't. A lot of things won't work if you do.

The AppleCare expert was misinformed. You already read the applicable documentation, in which there is nothing to support her claim.

Besides, it's working on all my Macs, and they all use FV. It's a red herring.

If all else fails, try "signing out" of iCloud on your various devices, and signing in again. That may be inconvenient, depending on the number of Macs you have. The last ditch effort might require un-pairing your Watch from the iPhone, and pairing it again. That's even more inconvenient, and I can't imagine it would be required, so exhaust all possible alternatives first.

As per this article : Automatically unlock your Mac with your Apple Watch - Apple Support

How to use Auto Unlock

Auto Unlock gives you instant access to your Mac computers when wearing your Apple Watch. Just wake up your Mac and you’re ready to go—without typing a password.

The first time you log in after you turn on, restart, or log out of your Mac, log in by entering your password manually. After that, Apple Watch logs in for you.

Auto Unlock works when you're wearing your unlocked watch and are very close to your Mac. If necessary, move your watch a little closer.

And if file vault is turned on automatic login is disabled .

Filevault is set to encrypt the hard disk , Apple Watch is used to unlock the admin password , there is no connection between file vault and auto unlocking .

Articles for file vault : Use FileVault to encrypt the startup disk on your Mac - Apple Support

: macOS Sierra: Encrypt the contents of your Mac with FileVault