In case someone reads this far, this setups is now up and running nice. In case someone wants to recreate this, I used the cheap Logilink UA0099 7.1 USB for this.
I ran into one more Problem: If you want to run MPD into the Loopback without an HDMI-Device attached on boot you should use index 2 instead of index 3 as I did in my start post. Somehow MPD does not like to have to play into device 3 when there is no device 2 present. Somehow bluetooth audio still came through fine. Well, just be aware.
