Re #5 how to reproduce with a normal Ubuntu installation:
1. apt-get install gdm
2. Choose gdm as the default display manager when prompted.
3. apt-get remove gdm
4. cat /etc/X11/default-display-manager
This should just say "lightdm" and following boots should fail until the file content is changed to "/usr/sbin/lightdm".
Re #5 how to reproduce with a normal Ubuntu installation: default- display- manager
1. apt-get install gdm
2. Choose gdm as the default display manager when prompted.
3. apt-get remove gdm
4. cat /etc/X11/
This should just say "lightdm" and following boots should fail until the file content is changed to "/usr/sbin/ lightdm" .