When I was stopped for over speeding near Sadiqabad (I was doing 75km/h on a 50km/h stretch), the warden only asked for my driver's license. I did have the original book, updated token tax, and the highway codes book. I did not have the 3rd party insurance with me.
There was a rangers check post right after we left Sadiqabad for Karachi, and they had stopped all vehicles. The guy asked where I was going, and whether I had carried a weapon for safety. Asked my wife to open the glove compartment (just a formality I believe) and that's it.
Road conditions are generally good with only a couple of patches near Pano Aqil and Ghotki where repair work was going on.
Fog was not an issue at all as I have left Sadiqabad at 8Am in morning for both up and down travel. Same was the case with Lahore which I left at 8AM, however, I heard that motorway was closed because of Fog even after 9AM, but I'm not sure which portions were closed though.
Since I did not use the M-2 and M-4 I can't comment on that route, however, I believe if Fog is not an issue, both these routes will be more comfortable compared to N-5.
Regards