@chrisc As I recall I had this working last spring or summer. Returning to it on a Nexus 5 today (following upgrade to the RC for OTA-7), the lightdm restart is only working for me if called by a root shell over ssh from another machine, not from a sudo -i in the built in terminal, which kills lightdm and gives me a black screen without lightdm ever coming up again.
I tried ssh-ing to localhost and running the commands from sudo -i in that ssh session, but that too does not result in lightdm coming back. I also tried using "stop" followed by a sleep and a "start" thereafter, but again, lightdm does not return.
Have you encountered this on any of your devices? Any idea what could be causing it, or what to try as a workaround?