Honestly, how can such a complete mess be made out of one very simple prosecution?
On another note, not being from the US I'm still not really understanding how this Grand Jury thing is apparently a safeguard against malicious or improper prosecution. I had no idea that it involved only the prosecution giving evidence to the GJ. How on earth is that a balance against impropriety if the other side can't chime in? I mean, the matters brought up in this video are surely precisely the things which a GJ are supposed to prevent, are they not?
The requirement for a grand jury is actually in the U.S. Constitution, it's part of the Fifth Amendment. That only applies to federal crimes, but it was also the model of a lot of state laws. Although unlike federal, in many states there are alternatives to going through a grand jury.
The Founders were concerned about some of the excesses they saw in Great Britain where the prosecutor alone would determine who could be indicted. So a grand jury was supposed to be a check on the system and ensure that the state had at least established probable cause before a trial was held.
You can debate how well it functions. It can certainly be abused, but so can any other system. I've known a couple of people who've been grand jurors and it's a lot of work and responsibility. It's not like a trial jury where you serve for only one trial. You actually have to be on the jury for months hearing case after case. My friends took their duties seriously and would ask questions of the DAs. They weren't just rubber-stamping the indictments.