Can other apps play music from my Google Music library?
They're only accessible from Google Play Music. It stores them somewhere in the internal space of the app, not in the music library of the phone. There is no way to tell the app to move the file somewhere else at the moment, and no way that I'm aware off to point other app to that specific folder.

That's the main reason why I don't use Google Play Music. I want to be in control of my music folder.