It looks to me that you were provided all the information you needed in your last thread on the topic:
https://yabb.jriver.com/interact/index.php/topic,118419.0.htmli.e. When using HDMI ouput, configure MC to the physical speaker setup you actually have, and MC will take care of the potential channel issues. Don't use “Source Number Channels” as the output in DSP Studio. Use 5.1 always, if that is your physical speaker setup.
Of course, if you want Stereo media to be played in stereo, the MC DSP provides the option to detect stereo and handle it appropriately, as an exception to the above 5.1 configuration.
You also drew the incorrect conclusion:
Got it, so I will assume that WASAPI is an unreliable choice for a one stop HDMI audio device.
WASAPI is fine with the above configuration.
Also take note of the warning Hendrik supplied:
None of those two alternatives are direct hardware interfaces [Direct Sound and Asio4all]. They probably convert the input at some point so that its actually output as 5.1. DirectSound does that for sure.
If you use Direct Sound or ASIO4All, your audio is probably being remixed between MC and output from your PC, which sort of defeats the purpose of using MC to play high-quality audio. You have no control over how Direct Sound remixes the audio, and I assume the same for ASIO4All (I don't have experience with ASIO4All).