I can verify that this is a CE problem. The same symptoms occur on my system (W7, Kodi Helix).
I can play music or a movie in Kodi and properly quit the program without issue using all methods. However, if I watch a movie using Cinema Experience, ExecBuiltIn(Quit) does nothing, and trying to quit using s --> Exit causes a runtime error ("This application has requested the Runtime to terminate it in an unusual way.")
I can still close the Kodi window if I use CE (as opposed to properly quitting the program), but that doesn't ensure the Kodi process is terminated.
All of this was tested without using VoxCommando.