In the same menu you using to get to safe mode, (start up options screen), maybe try enable low resolution video and see if that changes brightness.
I am curious if uninstalling graphics cards would reset this, it isn't a setting I use myself.
seems reinstalling display adapters might do the trick: http://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/windows-10-brightness/efa42447-d53f-417e-bd9c-e60e7d2408b5