How do I make root appear in my lightdm user list [duplicate]

I would strongly advise you against doing this, even with the compartmentalization you mentioned.

Why? Here's the problem. What if there's a problem with, say, Cinnamon, that caused it to accidentally rm -rf /usr? That exact situation has happened with multiple different programs. Please read this post on Ask Ubuntu, and make sure you understand the risks.

If you still feel like this is worth the risk, here is what you need to do:

1. Make sure root has a password (sudo passwd root)

2. For lightdm, edit /etc/lightdm/lightdm.conf. Add this line:

    greeter-show-manual-login=true
   

   For mdm, edit /etc/mdm/mdm.conf. Add this line:

    AllowRoot=true
   

Then reboot (or restart lightdm), and you should be able to log in as root.