Ubuntu default desktop manager
What I like about linux is that there are configurations. You should be able to manipulate the whole system without a gui. The configuration for your default display manager, for instance, is in the default-display-manager file:
> cat /etc/X11/default-display-manager
/usr/sbin/lightdm
There's a startup script /etc/init.d/lightdm that checks that the value stored in this file points to a lightdm program and then executes that program.