hi
1) since you want 2 devices.....you initially blacklist your internal then re-create it to load with index=1, there is no need to force usb onto index=0.........I have failed to explain that bit to you eh?
Pls post the output of your bootlocal and your grub kernel line bootcode for blacklisting
and post in a code box the command pls.
lsmodEDIT we will need the full list to see the usb stuff.
2) since I lack skills in mpd you may consider this link......tinypoodle or others may be able to mod it so you have
udev rules for usb sound device detected
then piping into both outputs at once.....while it appears to only swap at the moment
https://bbs.archlinux.org/viewtopic.php?pid=3349883) leaping ahead.....I not sure what your sound module for usb is called 
count the number of colour connectors......they are your stacks.
you have a spdif........not sure as I asked but maybe we got confused?.....pls specify if that is to be true output pls....so why the need for others?......I repeat.... which alsamixer setting are you referring to you for digital out....spdif ?
4) it might be useful to post a snapshot of your current alsamixer output screen?
5) it appears you may have only this to play with
Module snd-usb-audio
  --------------------
    Module for USB audio and USB MIDI devices.
    vid             - Vendor ID for the device (optional)
    pid             - Product ID for the device (optional)
    nrpacks        - Max. number of packets per URB (default: 8)
    async_unlink    - Use async unlink mode (default: yes)
    device_setup    - Device specific magic number (optional)
                    - Influence depends on the device
                    - Default: 0x0000 
    ignore_ctl_error - Ignore any USB-controller regarding mixer
                   interface (default: no)
    This module supports multiple devices, autoprobe and hotplugging.
    NB: nrpacks parameter can be modified dynamically via sysfs.
        Don't put the value over 20.  Changing via sysfs has no sanity
    check.
    NB: async_unlink=0 would cause Oops.  It remains just for
        debugging purpose (if any).
    NB: ignore_ctl_error=1 may help when you get an error at accessing
        the mixer element such as URB error -22.  This happens on some
        buggy USB device or the controller.
so I am guessing we could try....in your bootlocal.....optional add index=0 but I think thats unnecessary
/sbin/modprobe  snd-usb-audio  vid=08bb  pid=2902