Weellll. I tried it.
Removed both autologin entries from the lightdm.conf (autologin-user=user and autologin-user-timeout=0)and added "greeter-show-manual-login=true".
Saved. Restarted, and at startup it just goes into some strange mode where the screen is black/grey/black/grey... with the white mouse pointer the only thing showing, an endless loop.
Have to restore it to get back in.
Also tried removing another entry - user-session=une-efl and got the same behavior.
I tried to attach a screen shot of what is displayed at startup after restore, but attachments do not seem to work.
I am sure I am missing something and I am a total newbie in Linux but want so badly to use Ubermix at the school where I work.