i wouldnt hold my breath on the win10 streaming. from what i read i know it works for music, movies and similar but not so sure if it has a feature to stream while you are gaming. you could read up on it as i just posted it because i thought it be worth looking into to see what they will and will not end up supporting.
for the xbo i'd likely still use a hdmi audio extractor. its a tried and true method and hdmi audio would be uncompressed.
the trouble is not getting surround from all sources, thats the easy part. the problem is that you want 5.1 input into your pc. if you can live without the input into your pc so you can control the volume via software (or whatever you plan to do) and just hook the z506 up directly (through an audio extractor) you'll have alot easier time of it!
the xbo part is easy, lets concentrate on how to get 5.1 from your tv.
-provided your tv and xbo both support 5.1 out of the optical ports you could always get an audio extractor which has spdif input like
http/www.amazon.com/Panlong-Audio-Digital-Decoder-Converter/dp/B00AMC2J1Y (READ: verify it supports true 5.1 input and doesnt upconvert, the text reads funny in their description) and connect both non pc sources up to it. hdmi would be a bit better for uncompressed audio though.
-if your tv supports arc, they do make extractors which are arc compatible so you may be able to pull audio that way (check your tv)