How to Properly Uninstall Brother Twain Drivers in OS X 10.6
To answer both questions at once, it seems that the Brother instructions only cover system level files.  This will not remove any files that you as a user created or installed.  I would go through the instructions again, then repeat the instructions, but for every location where it says "Macintosh HD > Library > x", I would also check at "Macintosh HD >  Users > YourUsername > Library > x". 

It is also possible, depending on what version of OS (I see that you are rolling back to 10.6.8 - does that mean you were on 10.7?) that they are system installed.  I wouldn't recommend removing anything from your system folder, but it may be finding the drivers under "Macintosh HD > System > Library".