Thanks for the interesting feedback.
I don't know to what extent a difference it makes, because everything I see, I think is obvious (forgot the term for this): I think holistic and heuristical thinking is the de facto, and most obvious way of dealing with multidisciplinary problems; in this case, I believe this falls into the same category as the "wicked" problems you describe. It seems like everybody on this forum has a fair amount of exposure to psychology, such that many concepts that I learned in a formal setting are in fact common sense. I have no idea how true this is, but I try to avoid stating the obvious, so I generally keep quiet about it.
I still suspect the following would be nothing new to you, but it's a different route of thinking so I'll write anyway.
A problem that "cannot be clearly defined" and has "no clear answer" is generally solved heuristically. This is an area where hard-earned experience, and not necessarily genius, pays off. Warfare is a good example, because it involves not only logistics but also understanding the enemy's culture and psychology and estimating their intelligence. Heuristics come from efficient processing of declarative and episodic memory, which are, very obviously, the result of accumulated experiences across diverse fields.
So regarding how to best design a car, it is logical to think that an engineer would be better than an artist, because they know what is required to make a car work, but an engineer with an art background is better than one without, because they can efficiently (i.e. long-term memory retrieval) co-synthesize knowledge pertaining to design and construction. This must also be the reason why pg says "hackers who can design are dangerous."
Your points about forum dynamics follow the same ideas. A flat discussion, which interferes with thought organization, forces readers to synthesize information as a whole, which places a larger stress in keeping relevant information in memory. A threaded discussion, on the other hand, has less stress on memory, because certain steps can be stored as logical patterns that draw from existing memory.
A very simple thought experiment to illustrate this could be as follows. Imagine a threaded discussion with these messages: man. suitcase. taxi. airport. airplane. country. building. meeting.
Next, a flat discussion: man. window. smoke. lake. chocolate. tree. bulldozer.
Now paint a mental image out of the information from these two threads. The first follows a very logical progression, is easier to remember, and can be painted in a fairly predictable manner. The second one seems more disparate, requires more memorization, and the end result is harder to imagine, a la the "wicked problem."
Hope I'm not too far down a tangent... I guess the relevance to the diagram is, it doesn't matter what diagram or map you use; the goal is simply to get all the facts into everybody's heads simultaneously. A mind map could work. A list would work. A powerpoint. Or, based on the type of stimulus, a mixture of things (my interest).