Ok I traced the block diagram [1] and it looks like that the input mix option does do what you want (send mixed signal EXCEPT for USB to PC, and send USB to the main mix along with everything else). Confusing indeed but it looks like it works!

Note in this cause you won't get a fully separate aux mix, and you'll have to juggle the levels around to get a hot enough signal to your PC as well as the correct volume in your monitors.

[1] https://d1aeri3ty3izns.cloudfront.net/media/24/244329/downlo...