Make Ctrl+Alt+L show login screen

This is possible. Here is the procedure.

Open up settings and go to keyboard->shortcuts->system and unbind the Lock Screen shortcut. (Make sure it says disabled where the key combo normally is)

Go to Custom shortcuts on the same tab and create a new shortcut named login screen. In the command field enter the following:

gdbus call --system --dest org.freedesktop.DisplayManager --object-path /org/freedesktop/DisplayManager/Seat0 --method org.freedesktop.DisplayManager.Seat.SwitchToGreeter

Set the keybinding to Ctrl+Alt+L and then you are all done. From now on when you press that key combo you will be placed on the login screen.