From everything I can see in your log, it looks like the display is getting recognized fine, but it's not activated when you start the X server. You have several metamodes in your xorg.conf:
Option "metamodes" "DFP-3: null , DFP-0: 1920x1200 +0 +0;DFP-3: 1440x900 +0+0, DFP-0: null; DFP-3: 1440x900 +0+0, DFP-0: 1920x1080 +0 +0; DFP-3: 1280x800 +0+0, DFP-0: 1024x768 +0 +0; DFP-3: 1024x768 , DFP-0: 1024x768 +0 +0"
According to Xorg.0.log, it's selecting the first one by default, which has your external display off:
[ 13.207] (II) NVIDIA(0): Setting mode "DFP-3:null,DFP-0:1920x1200+0+0"
Does switching to a different metamode work?