I posted this in the Volumio blog, but it might get more attention here.
Just a shower thought: To solve #1 bug (i.e. network over USB), would it be possible to run network communication through an expansion board with ethernet and/or wifi?
This would circumvent the USB ethernet, right?
Fun idea, but I doubt if it is possible. You need to figure out a way to do the communications over the GPIO pins and I don’t think you can achieve a reasonable communication speed that way. If so, it would be an idea to create some kind of full USB host compliant board which can do proper USB communication and/or ethernet over USB.
Google didn’t find any such boards for me 
You can send 24/192 data via I2S to a dac, why not send the same data to a USB port? Maybe like this?:
sainsmart.com/raspberry-pi/e … f-use.html
I am not familiar with these expension boards you just mentioned. But when taking a quick look these boards I think they just add another USB hub (you have to connect a small board into an USB port of the RPi and the other end into the expension board) so even more ports to feed of the one original USB OTG port of the RPi. I don’t see how these boards solve the problem (but I might be wrong 
).
This board was just a the result of a very quick search. But according to this: http://arduino-pi.blogspot.dk/2014/03/speeding-up-i2c-bus-on-raspberry-pi-and.html, you can reach speeds of 400 kbit/s via I2C. I haven’t found a board yet that uses this for extra USB or ethernet, but I don’t see why it would not be possible. However, I am not an expert on these things!