So, update of sorts. The repair work on my camera is on hold for parts. Called Nikon support and the lady said they are most likely having to replace the power supply module board, but exact details will be confirmed later when they have performed and confirmed repair.
Thanks for the update!
Appears that D850 is exibiting same/similar "won't turn off" issue, see link below. The OP, doesn't know about this issue, but his description, points 1-3, matches the problem I have with my D500 exactly.
https://www.nikonforums.com/forums/...ical-problems-and-sent-back-to-nikon-factory/
Seems like this is a serious quality issue that Nikon are struggling to resolve?
That is annoying ...
You thinking about buying a D850 or already have one? I've been considering the D850 for some time. Now that I am "cameraless" my motivation to buy one has increased

. This is reason why I started researching reported D850 problems and stumbled upon the above thread...
JC is not going to buy a D850, he enjoys DX format.
The D850 appears to be a very solid camera, more so than the D750 or the D800. The D750 suffered numerous recalls, the D800 models with US serial numbers below 3055000 have a potential to need re-calibration of the AF system, and the D800 is more prone to being totaled if it is dropped due to a flawed design of its frame.
My next camera is almost certainly going to be a D500 or another D800/D800E and then a D500. The D850 is a great camera and could easily be the last camera you buy, so if you have the money I would definitely say go for it, I just don't have the money for that so have to make compromises based on optimum value.