You have two issues -
1. installing fglrx. Lets deal with that later.
2. external monitor resolution - you want external monitor to have correct resolution - you are prepared to allow the internal to be lower for the presentation?
The workaround suggested is resizing the resolution to 1024x768 for presentations. This is easiest but uncomfortable, especially if you do a lot of presentations.
The next thing you did not mention trying was adding modes to xorg.conf display subsection so X does not have to guess the correct resolution. Have you tried that?
I'm guessing your xorg.conf is different now.
Next time you have the external plugged in, open a terminal and try:
$ sudo dpkg-reconfigure -phigh xserver-xorg
Then check your xorg.conf to see if extra entries have been added.
this should give you a list of monitors (by output) and resolutions. If vga is not connected to anything it will just say so.
xrandr --output VGA --mode 1024x768
should do the trick - provided VGA is the external of course.
If you are using the radeon driver, then that will be vga-0
BUT, this is the same as adding modes to xorg.conf
xrandr --output VGA --right-of LVDS
Now it's dual head.
xrandr --output LVDS --rotate left
A decent guide for configuring xorg.conf for xrandr - see