In regard to the USB default app always being locked to one specific one in Android 6 and not giving you the option to select the app you want to use, I worked out how to restore this option. Go into Settings - Applications - Default Applications - Change Default app selection to "Ask before setting default apps" then each time you plug the transmitter in you will get the option to select which app you want to use as long as you click on "Just once"
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Sent from my SM-G920I using Tapatalk