Well, speaking just for myself (the other people you mention are a whole lot more knowledgeable on these topics), everything you just mentioned is far, far beyond/outside my level of knowledge/expertise, so I can’t help with any of that.
I plan to contribute photographs of results of mixing real media to compare to your MyPaint spectral mixing code results. Which I will do as soon as I get the Neopastels I ordered from dickblick.com (hopefully later today).
Unlike my current stash of student grade oil pastels, the Neopastels actually supply pigment information, so the colors can be coordinated with paint mixing information from the handprint.com website.
It would be really cool and helpful to have feedback from actual digital painters or even photographers who might be curious about “colors mixing like real paints”.
I’m guessing for a lot of people the first step - compiling your modified mypaint code - might require a step-by-step “how to”, or better yet if an AppImage could be put together.