Question about NVIDIA Shield TV Pro

Yes, it's possible, this is what I do. The USB out will become a USB Audio out automatically if you connect it to a DAC or Receiver

you don't need any additional hardware or software: the Shield will automatically split and route the sound to the USB and the video to the HDMI. Just ensure you have a USB Audio In on your receiver and you're set. In the Audio Settings of the Shield chose the desired Audio Formats and that's it. I route the audio directly to my DAC, nothing in between

No it is not possible. You need to purchase an hdmi audio splitter to extract the audio signal