The new E17K II does DSD so you can playback DSD DoP natively but I wouldnt be transcoding your sources to DSD.
For what I think you want to do, you may be best to set up two zones in MC21. One with dedicated 2 channel that can do DSD or whatever 2 channel source you want to play through your DAC and then stereo connection to the Logitech RCAs.
For 5.1 I would just use the realtek output from the PC using optical or if you must with the line outs and match the colour coding to the Z506 inputs. Set this up in another Zone in MC21.
Playback 2ch and DSD content in the E17 DAC Zone
Playback movies, multichannel in the Realtek 5.1 Zone
I would stay away from JRSS unless you want to upmix stereo to psuedo surround or downmix multichannel to 2 channel.
Each zone can have it's own upmix and downmix settings, so you have a lot of flexibility but I'd just leave 5.1 as 5.1 and playback 2 channel as just 2ch for now.
You could possibly get funky with ASIO and DSP channel routing to get 2ch from the the E17 DAC with subwoofer coming from the optical or analog inputs to create 2.1 but that's another whole advanced topic.
You probably dont even know yet what your best use case is until you understand what MC DSP engine and Zones can do.
I suggest starting out simple with what I suggested above with 2 completely separate zones treating the E17DAC and Realtek 5.1 as completely separate playback devices. When you get that working and understand how MC21 DSP and Zones work, then move on to learning more. It's just like the analogy, you have to learn to walk before you can run, and you're going into complex territory there cowboy!
You cant just walk in to Einstein and ask him to explain how to build a time machine without first understanding the laws of physics.
Hope you understand, you really need to take this one step at a time. It'll be more satisfying to get the solution working and understanding how it works. The wiki Jim pointed you at is a good place to start, do a search for Zones to understand how that works and do some reading.
cheers!