What you're experiencing is typical of Grand Juries. Prosecution doesn't have a duty to put on their whole case, or typically even let the defendant testify (Officers often are permitted to testify in their own defense at the GJ level, however). All a prosecutor has to do is put on enough evidence to secure probable cause. They bank on the quick turn around on each case because they have hundreds in which to indict - it's a mere formality to them. Functionally, today, they are right. That was not the original purpose of GJ's when they were written into the U.S. Constitution and many of the state's constitutions, it was to provide an extra layer of accused protection by way of their peers from governmental intrusion. I appreciate very much that you are taking your position seriously in this vein, and I suggest trying to push back and ask more of the prosecutor.
Because of the current procedure, a prosecutor can effectively indict any case they wish. Take the recent failure to indict arising from the incident in Ferguson. In all likelihood, a prosecutor would merely have to provide evidence that an unarmed man were shot and simply leave out the self protection defense claim in order to secure an indictment. Instead in that situation, as I understand it, the prosecutor did not believe there was a case. Usually when that happens, a prosecutor is obligated to not attempt to indict (and they really don't do so unless they think there is a case, in my experience). However, under great pressure to attempt indictment, the prosecutor in Ferguson took the unusual step (though completely permissible) to present all of the evidence he had in the case and truly leave it up to the discretion of the GJ. This of course ignores the statistic (dare I say fact?) that police frequently get some sort of deference in GJ indictments.
Additional little fact most folks don't realize, a failure of a GJ to indict does NOT bar a second, third, onward ad nausea attempt at indictment in succeeding GJ's. Jeopard does not attach until a jury is sworn in at trial or the prosecution calls their first witness (in a bench trial).