Me too! Just call me Gumby.
No, I think most of the people in a position to do something just didn't see it for what it was. Just like we see people here at WS who simply refuse to even consider it. Some of the people in the DA's office, knowing the person couldn't be charged or even named as a suspect, may have thought going after the people who were ultimately responsible (parents), and who had committed crimes in their attempt to conceal what happened, was the correct way of pursuing it. Others may have become suspicious of one or both of the parents because of the way they acted afterwards. Still others may very well have helped out (not even knowing or caring who did it) because of business or political ties, or the hope of their careers in the future if they "went along".
As far as the Karr fiasco... Did I mention yet today that Mary Keenan Lacy is simply an idiot? If not... Mary Keenan Lacy is an idiot! (but that's JMHO).
Why on earth would you think that hard to believe?
I think you named the reason: Arrogance! I'd also add to that: Narcissism, confidence in their (JR's actually) ability to fool people, and their need to continue trying to explain themselves to the public. (Not to mention also the fact that they were paid handsomely for their books.)
And on that, we can certainly agree.