It seems to me that the CUVID is incorporated into the LAV Video Decoder, correct? I have it set to RO-HQ and when I right-click to check the filters I just see the MadVR and LAV Source Splitter.
Since the LAV Audio/Video decoder is automatically configured, then that should mean the CUVID is active as well. In checking with the Nvidia Inspector I am seeing that the VPU is about 45% while CPU usage is under 10%. Playing a 1080p 24 video source as my test video, it looks like CUVID is working.
I guess what I can do from here on out is do all of my media playing in MC, and just do my TV viewing (Ceton) on WMC. Setup a shortcut on the Harmony to open MC (after closing WMC first) so should be good for the time being.