I don't know if anyone else will have this issue, but I'm posting in case I can save some time for someone else trying to figure this out.
Problem: How to have your mac restart AND start up the Plex Media Server (PMS) while you're away and at the same time keeping your account locked up on your mac.
My example: I'll be traveling for a month with a house-sitter staying at my house. I want my house-sitter to have access to the PMS on the house TV's, as well as myself via my Roku stick. But I don't want my house-sitter to have access to my computer. This also needs to start up automatically in case of a need to restart the computer.
Overview: Create a user just for Plex. Move your library to that new user, including all your preferences. Make sure your mac is set to restart after power outage, auto login to Plex user, sleep after 1 minute, then require password to wake from sleep.
First create your new user, set this user to auto login.
Copy ~/Library/Application Support/Plex Media Server/ to somewhere that will be accessible to the new user. I put mine in /Users/Shared/Library/Application Support/ in case I wanted to just symlink to it from both users.
Copy ~/Library/Preferences/com.plexapp.plexmediaserver.plist to somewhere accessible by the new user.
Start your new plex user. Start Plex Media Server, which will result in an empty library. Then QUIT PMS and either replace the new /plex/Library/Application Support/Plex Media Server/ with the one from the shared user, or just create a symlink. Also move the plist file from above into a corresponding location for your new user.
You will then have to play with permissions to allow access for your new user. For me, I just added the new user to that new PMS folder as read & write and copied to enclosed items. I also had to make sure the new users had access to any drives where library media was stored.
Then I restarted and everything works.
Don't forget while you're in the new user to set up your system prefs to wake from sleep on LAN, restart after power outage, monitor sleep after 1 minute, and require password to wake from sleep. Also set Plex to startup on login.
Also, don't forget to go into your old user, and deselect PMS to auto startup. It won't work anyway because it should already be started under your plex user and you can't run two instances on the same mac. But better to just delete it from your start up list.
Just as a post-script, I also added a script to keep my mac awake while streaming from Plex. I found it on a forum somewhere, so I'm sure you'll be able to find it if you find it necessary.