Nikon D800 Not Detected for Tethering - LR 5.3 - Windows 7 / 8.1

Question

Win 7 Machine: Homebuilt, Lightroom 5.3, all drivers and windows updates up to date.
Win 8.1 Machine: Dell XPS 15 (2013 High Res Touchscreen edition). Lightroom 5.3, all windows updates applied. Drivers up to date.
Camera firmware up to date as of Jan 2014. (A, B and L all at latest versions).
On the Win7 Machine, the camera shows up in device manager, can be browsed in Windows Explorer and works with Nikon Camera Control 2.13. It does not  work in Lightroom.
On the Win8.1 Machine the camera shows up in Device manager but with a yellow /!\, it does not show up in Windows Explorer, it cannot be detected by Lightroom and Nikon Camera Control does not work.
Nikon Technical say they've tested against their Win 8.0 and 8.1 machines and it works fine. Despite being very friendly and sympathetic, this was about the extent of their help. They confirmed that there are no drivers for the D800 and that the OS should install the generic USB device driver, as it has done so on the Win7 machine.
Camera firmware is up to date, camera batteries are fresh, all tests have been conducted with and without a memory card in the camera. I've used a USB2 Micro cable, a USB3 Micro cable in both USB2 and USB3 sockets, as well as through a powered USB3 Hub and an Active USB3 extension cable.
All permutations work fine with Nikon Camera Control on the Win 7 machine.
None of the permutations work with Lightroom.
None of the permutations with with either Lightroom or NCC on the Win 8.1 machine.
Tried changing the USB3 routing mode from EHCI to XHCI and that didn't change a damn thing either.
Any input gratefully received, I'm flat out of ideas and very frustrated.
Capture One 7.2 works flawlessly on Windows 7, but their UI is lightyears behind Lightroom's so I'd rather get things fixed than jump ship.

Answer 1

Hi Denyerec,
Since not even the Nikon software will tether the camera for you, it sounds like whatever problem you're having is at a much lower level than Lightroom (at least on Windows 8.1).  Tethering with the D800 is supported by Lightroom on both Win7 and Win8.1 (and works fine in my tests).  You can find some tether troubleshooting tips here:
http://helpx.adobe.com/lightroom/kb/troubleshoot-tethered-capture-lightroom-4.html
It sounds like you've tried a lot of that already, though I would definitely suggest trying a different USB cable if you haven't done that already, as that can make a big difference.
You should also check to see if the tether process is running, like so:
1.  Connect your camera and turn it on.
2.  Launch Lightroom and initiatate tethered capture.
3.  Start Task Manager and look for a process called "tether_nikon.exe *32".
Let me know if any of the troubleshooting steps help, and let me know if the tether process is running.
Thanks,
Ben