1) I had music playing and I backed all the way out to the sign in screen and signed out. Music kept playing. I went up to the now playing music note at the top to stop it and it didn't do anything. I had to back out again, sign in, then I could stop the music. Sign out should automatically stop everything that's playing/paused.
2) Emby is very difficult to exit. I mean, the only way to stop it is to go to the main menu of the Xbox, push an option key, and kill it. Emby should behave like the YouTube app: if you try to back out further than than sign in screen, it should prompt if you want to close the app.
3) The lists of songs in the app never show their favoriated status on a 1080p TV. The only way to see the favorites is to play each song individually and see if the heart is checked. This likely goes hand in hand with a similar issue on web UI where the screen has to be ridiculously wide for the favorite column to present.