I have finally got a "Play" button now in my games section in Emby Theater with the extensive help of @crazybbs however, now I have discovered a new issue (I searched the forums and could not find any threads reporting this issue, so here goes).
When using Emby Theater to launch a game, Theater minimizes, and the game launches. However, when playing the game, Theater is still reading the controller commands (it makes an audible 'bloop' with each button press, just as when navigating Theater)
If this is this a known issue, I apologize for double posting. If I simply have something setup incorrectly, can someone please help!