Audio doesn't pass through a DLNA Controller. The Controller just directs the Server to send the audio to the Renderer.
So either the audio is the same as when using another Controller, or you have your setup wrong somewhere. As you are running a DLNA Server on the NAS, have you imported files into MC via the NAS DLNA Server, or do MC records point to the file location on the NAS correctly? Basically, using a DLNA Server on a NAS with MC is a bad idea. Much better to use MC as both the DLNA Server and Controller, and send the audio from there to the Renderer... unless you turn off the PC that MC runs on regularly, and still want a DLNA Server running for other devices to use.
But when using a NAS DLNA Server, even if transcoding is turned off, I think there may be some audio manipulation based on what the advertised capabilities of the Server and Renderer are, which the Controller would have to take into account.
There is an Android version of MC in development. You can buy it now, but it is in early Beta.
https://yabb.jriver.com/interact/index.php/topic,119740.0.htmlYour Thumbnails should be there now, but if the reported problem above is happening, then they should come back when you stop MC doing anything and refresh the view, while it is connected to the NAS. But if you are using the NAS DLNA Server to provide media information to MC, then all bets are off. It could be a problem with your NAS DLNA Server.