There's actually a HUGELY simple fix for this.
The error message given indicates that xorg.conf is failing to validate because "Screen" doesn't reference a device, basically.
You can fix this by adding this line to /etc/X11/xorg.conf, under Section "Screen": Device "Default Device"
(where "Default Device" is a name already used in xorg.conf, under Section "Device")
After adding this single line, nvidia-settings can save to xorg.conf just fine.
There's actually a HUGELY simple fix for this.
The error message given indicates that xorg.conf is failing to validate because "Screen" doesn't reference a device, basically.
You can fix this by adding this line to /etc/X11/xorg.conf, under Section "Screen":
Device "Default Device"
(where "Default Device" is a name already used in xorg.conf, under Section "Device")
After adding this single line, nvidia-settings can save to xorg.conf just fine.