Since you haven't enabled the root account, you are in a bit of a sticky situation.
There are several ways of fixing this, all of which require you to boot into single user mode.
To boot into single user mode, you will have to hold down Apple-S on boot.
When you do this, you are taken into a command prompt when the system is in it's lowest init state. You will now have root-privileges, and can exactly what you want.
The first thing to do is to mount the filesystem.
% mount -uw /
When this is done, the easiest way is to delete the netinfo database, and rebuild it. Other options would be to write directly to NetInfo using nicl or niutil. Another option would be start netinfod, and indirectly enabling the root account with passwd root. But, the easiest is to simply delete your netinfo database, and let it rebuild itself.
% mv /var/db/netinfo/local.nidb /var/db/netinfo/local.nidb_BAK
This doesn't delete your netinfo database, it simply moves(renames) it.
Now, to respawn your netinfo database, we will use the Apple Setup Assistant.
% rm /var/db/.AppleSetupDone
Notice the punctuation before AppleSetupDone.
These 3 lines will sort you out.
Now you just restart your machine.
% shutdown -r now
When your machine now restarts, it will start the Apple Setup Assistant. This is the same thing you saw when you first installed OS X. You will again have to setup the primary administrative user.
The easiest now would probably be to name your user 'nathanad', and everything would revert back to normal. You could name the new user 'nathan', but then you would have to do some more Terminal commands, to move your old userfolder to your original user(or log in as root).
Do this, and get back to us if there are more problems.
PS: The reason why everything failed after you edited NetInfo is because when you changed your short-username you where moved out of the administrator group. There is still a user in the Administrator group named 'nathanad', so when you now logged in as 'nathan', you didn't have admin privileges. Or the user 'nathan' wasn't/isn't in the Administrator group.
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