Problem solved! In a nutshell:
First problem with the grey dot was a simple mistake of exporting selected patches and not all patches easy mistake to make!
Second problem, because of the adjustment stack on the displacement layer you were never exporting a 50% grey image, took me a while in the video to notice, another easy mistake to make.
Watch the video 
I get a render out that works just fine with your displacement. Small colour error in displacement equals a large headache at render time!
https://www.youtube.com/watch?v=HrI68u-n9QM
All the best
Dave
From a readers' Q and A column in TV GUIDE: "If we get involved in a nuclear war, would the electromagnetic pulses from exploding bombs damage my videotapes?"