I have the "push tracks to phone" capability working now.
I tried again today testing with the BubbleUPnP App on the phone which worked every time, showing up in as a DLNA Renderer in MC to be able to push tracks. I also tested using the mConnectLite App, which wouldn't show up as a DLNA Renderer in MC to be able to push tracks. It would only connect to a MC Server and then play tracks locally or to a MC Renderer, using the App on the phone. It also always played to the default "Player" Zone at that, regardless of the current Zone of the Server/Renderer.
First lesson:
The BubbleUPnP App on the phone is still running in the background once you close it, as evidenced by the notification with controls on the phone. This seems to work in a similar way to the new JRiver for Android notification. This still running part of BubbleUPnP may have been preventing JRiver for Android from running in my initial tests pushing tracks to the phone. You need to close that part of BubbleUPnP via the notification before switching to JRiver for Android.
Second lesson:
JRiver for Android uses a pure DLNA connection, I think, and doesn't use the additional magic of a MC Client/Server connection and settings that, say, a Windows installation uses. I guess that is self-evident in that there are no Client settings in JRiver for Android. I just expected JRiver for Android or the MC Server installation to query the capabilities of the phone, and send tracks in a format that would work. Nope. So, you need to associate a MC DLNA Server with the JRiver for Android Dynamic Zone that appears in the installation of MC you are using to push tracks to your Android device, and make sure all the conversions are set correctly so that the phone can actually play the tracks. For me, that was Original format for FLAC 44.1/16 and MP3 tracks.
This could be a trap for other users, as JRiver for Android acts more like any DLNA Server/Controller/Renderer, rather than another MC installation.
I don't know what the issue actually was yesterday. It could have been that BubbleUPnP was still running in the background on the phone, and interfering with JRiver for Android playback. It could have been the DLNA settings. More likely it was a combination of things, and the need to restart MC on the PC and the App on the phone after making DLNA changes. I restarted the applications a lot to get it working.
It is interesting to note that once I had it working, closing the App and closing the notification, then pushing audio to the phone still worked, and re-instituted the notification with audio controls. So I guess the JRiver for Android service keeps running in the background, at least for a while. It seems that the notification gets reinstated after a while with nothing being played as well. Perhaps because the phone is still connected to a MC Server on my PC?
Now that I have that working, JRiver for Android will become a little bit more useful for me, so I'll have to start reading the Release Notes and other stuff to work out what functionality is supposed to work now, and what is still coming.