How high is "high-IQ" and how low is "low IQ" in your book?

I don't have specific ranges in mind, but I think I'd call grad-student level sufficiently high-IQ.

smart outsider coming in and overturning a well-established scientific principle

Not necessarily overturning a principle, but rather opening up new directions to expand into. How about Woz, Jobs, Gates, all that crowd? They were outsiders -- all the insiders were at IBM or, at best, at places like Xerox PARC.

Almost every aspect of modern life, even for a polymathic genius, is going to be at least partially mysterious

Of course, but you don't trust a process you don't understand. You trust either people or the system built around that process. If your doctor gives you a pill to take, you trust your doctor, not the biochemistry which you don't understand. If you take a plane across the Atlantic, you trust the system that's been running commercial aviation for decades with the very low accident rate.