You should uninstall BOTH codecs, re-booting in between uninstalls. Then re-boot AGAIN after uninstalling the second one.
DO NOT uninstall just one, and DO NOT uninstall both without re-booting in between. Trust me on this.
When I researched codecs I ran into discussions involving people having issues because they did not uninstall both, or short cut the process by skipping the interim re-boot. When they finally did it "right", as I suggest here, their problems generally went away. I think this was mostly issues with iMatch but any app that uses them would have similar problems with "ghosts" left behind.
The double uninstall with boots between "jobs" prevents errant objects from getting "stuck" in Windows because they were locked by something. And codecs are "automatically" locked solid by Windows. The idea is to do everything reasonable to make sure you are fully cleansed of both codecs before re-installing the codec of your choice.
I really can't recall if Nikon apps need the codec installed separately and on a quick look do not see it listed as a requirement. If the thumbnails on the light table displays don't properly show the image thumbnail then you need it.
The codecs are mainly used by Windows built in apps. Windows Explorer, Media Viewer, etc., use them to display proper NEF thumbnails.
Some apps, like iMatch, have also used them. With iMatch it is optional but highly recommended by the developer (long story not important here).
The main advantage of a 3rd party codec pack is that you take care of all the major camera maker's codecs in one shot. Otherwise you would have to download and keep current Nikon's, Canon's, Sony's, etc. One stop shopping if you have the need.
A *possible* secondary advantage is that some codecs claim to be faster and more efficient, or users make that claim, which is why I tried FPV. I have some performance issues with iMatch that I wouldn't mind mitigating although I don't think FPV really helped in my case.
There might be other subtle benefits. But if you only want access to Nikon raw files then it is not clear to me what *obvious* benefits you get from a 3rd party codec. I have not had any problems with ViewNX2 or CaptureNX2 using FPV but I have no experience at all with your codec.
I know I'm raising more questions than answers but I guess I'm trying to say that it shouldn't hurt to stick with Nikon's codec unless you have a very well defined and compelling reason. And in your case, having problems with Nikon's apps, if it were me I would first do the clean uninstall and then install Nikon's codec, just to make 100.0000% sure that your 3rd party codec is not the problem.
Also make sure you have current versions of whatever you install. The WIC codec concept is fairly new so it went through the usual teething process. One advantage of Nikon's is that it is free, as well as continual updates.