Correct. All that has been established is Probable Cause. The prosecution will still have to prove its case at trial to the jurors beyond any reasonable doubt which isn't easy and a much higher standard than Probable Cause.
You place way too much faith in the legal system. By your own words, if AJ is found not guilty, it's not because he could be innocent, it's because the prosecution failed to prove it's case. There is a flip-side. AJ could be found guilty but in reality be innocent. Innocent men have sat in jail for crimes they didn't commit before, so it's certainly a possibility.
My problem with your arguements is that you argue from the POV he is guilty. You've already predetermined he committed a heinous act. If he is indeed guilty, I hope he pays dearly, but the door is still open that he is completely innocent. A GJ indictment does not mean he's guilty. It doesn't mean that from this point forward he should be presumed to have committed the crime and if not convicted, it's not because he's innocent, it's because the prosecution failed. You can spout all you want about "charges wouldn't have been filed" if there wasn't something to the case, but like I said above, innocent men have been convicted before, and I'm willing to bet more will be in the future.
And please save the arguement of evidence. Evidence can be open to interpretation. It's rarely black and white, close the book, he's guilty type of evidence that's presented to a GJ. If you have that type of evidence, you don't need a GJ. If you have that type of evidence, the defendant is more likely to accept a plea agreement because they know their goose is cooked.
This isn't a case of, "Well the GJ indicted hime, he must be guilty. The only way he gets off now is if the DA screws up." We're still in the stages where due process is at work, and until a trial jury says "guilty", he should be presumed innocent. That's how the law is supposed to work.