I believe the problem is that you're missing one line in your code.
So how can you edit lightdm.conf again ?
Command line has a wonderful text editor called nano. You can learn more about it later using man nano, but for now, log into tty1, like you already know, and do the following:
sudo nano /etc/lightdm/lightdm.conf
Add that [SeatDefaults] line and press Ctrl+O and it should ask you "Filename to write: /etc/lightdm/lightdm.conf ", so simply press enter. It should say [Wrote 2 lines]. To exit nano press Ctrl+X.
*N.B.: Ctrl is symbolized as ^ in the command line, so you'll see that a lot, e.g. Ctrl+x = ^X.
Now you should have exited nano, and can reboot using
sudo shutdown -r now
Personally I prefer nano instead of gedit, but that's just a matter of taste and what works best for you.
lightdm is supposed to be a light display manager, but if you don't mind using something else you can download gdm display manager which has it's own login screen (or greeter, in the jargon or linux world). Actually, this is the same path i took when trying to solve the same problem. So how do we get it?
apt-get install gdm
I don't exactly remember whether it should prompt you to set gdm as default display manager or not, but in case it doesnt, you can always do the following:
sudo dpkg-reconfigure gdm
Next you should see a prompt explaining what the display manager is, with thingy at the bottom. Press enter. Now you should see a screen with two choices lightdm and gdm, and using arrows highlight gdm, and press enter. Reboot using the same command as before.
Reboot with the prompt that i mentioned before.
So after you got to login screen and back to your desktop( oh and you can actually change desktop environments from login screen, too, just in case you might need that ) you can open gedit again and change that lightdm.conf file again, or you can live on with gdm. Your choicese. You can always get rid of either desktop manager by using
apt-get remove command. For lightdm that would be
sudo apt-get remove lightdm.
BONUS: If you're like me, and mess with desktop environments every once in a while, you might wanna kill boredom while fixing your problem. moc player is command line music player (yup, that's right), which you can install using `sudo apt-get install moc
And to run the actual player run mocp command. Also mc , aka midnight commander file manager is useful to have if you don't have access to graphical envirnoment. Again, sudo apt-get install mc is how you get this tool.
And finally elinks or links internet browsers are nice to have in command line if you have messed up gui and need to look up some answers. Again, sudo apt-get install elinks, run elinks, and g to enter the address you need, like askubuntu.com
Let me know if this helped a bit.
Some more info here: