Can't turn off Filevault

I reviewed your other Discussions. As I understand it, a User Account seems to have disappeared for reasons unknown, and you are attempting to recover that Account for the sole purpose of turning off FileVault for that User.

• Can I get the original admin back and turn it off from there?

The quick answer is "maybe". Perhaps for some reason that User Account became hidden, and if that's the case you can un-hide it. Determine if that's the case with

dscacheutil -q user

The quick answers to the other two questions are "no"... not unless you provide a valid FileVault key to unlock the startup disk. You can't get anywhere without that. Temporarily bypassing the FV lock for a subsequent login as you described earlier (using fdesetup) won't help either, since you need to start with a previously unlocked disk, and it would have been unlocked using a key from a different User Account. I don't believe that will be productive.

A request: for this limited purpose kindly reply to this Discussion, not the others you started. Creating multiple Discussions is OK because I understand you are also seeking answers to related questions. If other site participants choose reply to them that's great but I don't want the burden of following and having to reply to multiple Discussions. This Discussion may lead to possible alternatives, or not, I don't know, but first determine if that particular User Account became hidden. There may be an easy fix.

I doubt it though, because a User Account shouldn't spontaneously become hidden or disappear altogether. That symptom hints of media corruption that can't be fixed. The ideal solution would be to restore a Time Machine backup.

Would it be beneficial to rename an alternate admin to replace the folder with the contents of my user folder.

It's worth a try. The Apple Support document's warning is applicable only to those who don't follow its instructions. What you're proposing seems reasonable... given that you really have nothing to lose. I still think the underlying problem is a corrupted startup volume.

Post the output of

dscacheutil -q user

I know it's long. You'll probably have to use the "text" icon (looks like a sheet of paper). Or just look for the potentially hidden user on your own. If that's the "easy fix" I alluded to earlier the solution will be obvious.

What user should I be looking for with that command?

The User Account that went missing. Specifically, its "short name". If it is not present, then it did not simply become hidden, and that's it for the "easy fix" I've been alluding to.

What do you recommend for that if I am getting errors?

Replace the defective drive. They can't be fixed.

I looked in your other Discussions for a description of those errors. I seem to recall you saying something about it but it's difficult for me to find. If Time Machine is reporting errors it's more justification to conclude hardware failure is the cause of an inability to back up, as well as the spontaneous User Account disappearance that seems to be the subject at hand.