When thinking about your three primary questions in the past, I found some of the best theoretical answers on one of the first places I visited: the Wikipedia page on cartography:
The ancient Greeks and Romans created maps since Anaximander in the 6th century BCE. In the 2nd century AD, Ptolemy…en.wikipedia.org
That said, I believe that the questions you raised have empirical answers as well. Those answers will emerge from scientific inquiry: carefully controlled experiments that test variations in map design (independent variable) against map readers’ mastery of a space or concept (dependent variable).
I’ve only been in the systems thinking/mapping community for a few years, so I’m no expert, but my impression is that, for questions like yours, the community heavily favors theoretical answers over empirical ones.
In my ideal world, a balance would be struck, allowing systems thinkers/mappers to dream up new answers to their questions while rigorously vetting answers that have been proposed.
My big idea for Kumu is to answer a fourth question: How can system mapping software help systems thinkers/mappers strike this balance?
I think one option would be a user interface that allows map contributors to ask questions about map design, float and solicit theoretical answers, and run A/B tests with map readers to vet those theories.
My sources, loosely referenced:
Better, a nice reflection on performance in medicine, the industry that I feel is the leader in the race to balance theoretical and empirical answers
The Roadmap to Building a Data-Driven Optimization Team, a practical guide from Optimizely (which I believe Kumu uses) on
Science can answer moral questions, a TED Talk that pushes the limits placed on scientific inquiry
Growth Mindset, a short video from Khan Academy giving examples of scientific inquiry in youth education. Sal Khan’s first and second TED Talks are also excellent. Side note — Khan Academy has one open patent application, titled “Systems and Methods for Split Testing Educational Videos,” leading me to believe that Kumu could pursue intellectual property in this space as well.