Okay, after backing up your existing xorg.conf, I would try replacing the entire existing 'Section "Screen"' with this:
Virtual 1680 1050
I'm removing some options that you probably don't need. Assuming you only want to use 24 bit color and you want your built-in screen to always use its native resolution, you don't need the other "Display" options or the other modes listed for this option. If you remove entire existing "Screen" section (everything from 'Section "Screen"' to 'EndSection'), and replace it with this, it may work. End the session and log back in to find out.
If that doesn't get you working, we're starting to get into stuff I don't have direct experience with, as I haven't done extended desktop on an Intel graphics chipset personally. Something else you could try:
Add 'Option "MonitorLayout" "LFP, DFP"' to your "Device" section, i.e.,
VendorName "Intel Corporation"
BoardName "Intel 810 and later"
Option "MonitorLayout" "LFP, DFP"
If that doesn't work, we may need to get some advice from someone else around here. You may also want to take a look at xrandr, if you haven't already.