File Vault doesn't usually give the problems that you report, although your own situation - slow to finish, etc, might be normal (you do have to be patient
). But, it will modify certain functions in the system, such as the presentation of the login window. That's why I asked about what the File Vault pref pane reports now. Either FV is on, or it is off. If it still reports "On" or enabled, then you may need to take care that in other ways. (It's really one of those times that a simple reinstall of the system will be relevant, and effective. The simple reinstall is similar in function to a Windows repair install, if you have ever done that).
If File Vault shows"Off", then you also need to check the "Find My Mac" setting, to make sure THAT is off, too, as that will also affect how accounts are displayed (and that the Guest account will be disabled with Find My Mac turned on.)
I seem to be drawn to musicians
. You are third active band member that I have had occasion to respond with over the last several years. Latest plays keyboard (and misc others) with the Security Project (does early Peter Gabriel Band stuff), some members also have "history" with King Crimson. Maybe you have heard of them...
Point being - I like trying to help out, particularly as you "un-learn" some of the things that might be second nature for you on other operating system, but don't have a lot of relevance on a Mac - such as the need for antivirus software. You already have found that some AV apps might give you more problems than value.
Most Mac users don't like the performance hit of full-time antivirus software, and some work too well, as you have discovered to your disadvantage. Again, you may get less value than the narrow results of virus "protection"