Looks like it's mostly the order of the json string that was off (id before spec) as the commands themselves look valid against V6, not sure if there's a good json reader for QT as right now the code is based on string matches which isn't perfect (really would prefer JSON objects) so we can improve that later but first fix
Here's the changed libsrc/xbmcvideochecker/XBMCVideoChecker.cpp for testing , can just copy it in and compile.
Haven't got Kodi around right now for testing but can do that later.
// Update
Added missing brace in one of the commands, they only have examples for V4 but those still look fine the way we use it at least:
http://kodi.wiki/view/JSON-RPC_API/Examples
{"jsonrpc": "2.0", "method": "Player.GetActivePlayers", "id": 1}
which has the id listed last there instead of before jsonrpc
[MEDIA=gist]RickDB/63059e52848511fdd5f7a035de2b5a84[/MEDIA]