I have been using the default Samsung music player on my phone.
I guess the lines of communication got crossed there, oops. Sounds like you're having luck with GMMP now, glad to hear.
...there is no currently way to set the default music player in Android (at least there isn't in 10.1).
No, I don't see one in 9, either. I've never had a conflict with the players, though. GMMP is configured to start playing when it makes a Bluetooth connection, and it always starts once the phone connects in the car. At worst, you might have to turn off a similar feature in the other player.
I don't do reverse syncing as I tend to have all my music on my computer and anything I purchase is done on there. So all new tracks are on my PC first and then I sync my phone to my PC.
Reverse sync captures play counts, ratings, and playlists that changed on the phone since the last wifi sync. It doesn't sync back audio files.