Can other apps play music from my Google Music library? I have uploaded my song collection to Google Play Music, and use my phone to listen to them.

Now, if I copy the original music files directly to my phone storage, I can then use any app I want to play them. But what about music synced via Google Play Music?

If I download the songs to my phone in offline mode, can any other app be used to play the same collection (just like with the files), or are they only accessible by Google Play Music app?
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.