If you gave up then you cannot confirm if this solution works for you or not. But I will post anyway in the hope that somebody else may benefit from it.
Please, try renaming the directory
Do it from a text terminal or if you can use gui (more of this latter) do it from a different session than the user session being fixed. Then login again as the affected user. Did the problem solve?
My system has 4 users. Two users are members of the admin group and can use the sudo command. I upgraded from 13.10 to 14.04. After some time using 14.04 the second admin user started to have the same problem, from login at the greeter to a background image with a mouse pointer and nothing more, no menu bar at the top of the screen and no unity launcher to start programs, also CTRL+ALT+L didn't work anymore, to close the session go to another tty and use
sudo restart lightdm was necessary.
The affected user can use Gnome 3 without problems, only Unity session was affected.
As all others users could login using Unity or Gnome 3, It was obvious that something got corrupted in the side of the affected user. I wanted to fix that session without having to delete the user and recreate It. So I renamed his .config directory in the hope that inside It a configuration file was causing all this. And It was exactly that. The next login attempt into a Unity session was OK (but all programs reseted to their defaults). If I put the original .config directory in place, problem reappeared. Then I started to work in the .config directory, first I renamed half of its subdirectories (there were about 40 subdirectories), and in the first attempt I'd hit the group that contains the culprit. Then I'd put in place half of that half, and problem reappeared. Then I renamed half of that last group and... I think you already know how this goes on.
Using this method I eventually hit the culprit. Rename dconf directory solved the problem.
.config/dconf contents (actually a single file) does not necessarily have to be corrupted. Maybe it is not corrupt (at the filesystem level, my filesystem is clean) but contains invalid data that ended there because of some bug in who knows which component of the system.
Also I noted in your case all users are affected, so this may not be the solution to your problem. But I think that It's possible that all users experience this at the same time and still may be caused by the same file (at each user .config directory).