Unless the S2 can output digital audio over USB if/when it gets the Jelly Bean update (or via a custom ROM), I don't think it will work. Have a look in the Android Phones and USB DACs thread to see if anyone has had any luck using external USB DACs with the S2. That should give some indication.
http://www.head-fi.org/t/595071/android-phones-and-usb-dacs
Here's one post to get you started on your research (it's not promising though):
http://www.head-fi.org/t/595071/android-phones-and-usb-dacs/825#post_8863240
I used "S2" as the keyword to search within the thread.
Again, sorry if I'm being dense. Is it possible to use a converter which has single 1/4" female at one end for a standard headphone plug and converts that to 2x XLR (or mini XLR) plugs? One of your pictures is showing 2 on 1 end and 1 on the other, however I can't tell what's connection type is at each end. Most XLR converters which I've seen online are mono and won't convert a single stereo 1/4" jack into 2xXLR mono plugs.
I checked the back plate configs on the blog and it looks like it will be used for input only. However I know that they are still finalising design whilst getting the prototype plates in so things might change...
The SPDIF out is a really nice feature since it will make the USB only source device (e.g. Galaxy S3 if it works with the M8) compatible with any DAC with optical/SPDIF input which should make for a very versatile setup, e.g. auditioning other DACs when at a meet. I really hope he's able to keep that feature on all permutations of the M8.
Thanks for the pictures. It looks like the mini XLR locks into place just like regular XLR does
I can see plenty of mini XLR to ??? items for sale online so should make it easy for connecting with unbalanced setups whilst still locking in securely.