Thanks for posting, and I would be happy to help with your new docking station!
Thank you as well for the excellent detail you provided, that is much appreciated.
In short, the problem is the cable you are using to connect the NEC display. The cable you have with a DisplayPort connector on one end and a HDMI connector on the other end is a passive cable that is only designed to work in one direction, from a DisplayPort video output to a HDMI video input. It will not work in the opposite direction as you are trying to do.
Given the NEC display also has a DVI input, the most straightforward way to connect it to the dock would be to purchase separately an inexpensive HDMI to DVI cable such as this → http://amazon.com/dp/B014I8UQJY
The HDMI end of this cable will connect to the dock’s HDMI video output, and the DVI end of the cable will connect to the DVI input of the NEC display.
The other reason for purchasing the UD-3900 is that the mini-diplayport output on my surface pro appears to be faulty, interference/red line son dark areas, and I have been told a minimum £250 for any diagnostics and repair !