To my knowledge, no one here has ever said grand juries decide innocence or guilt. They do decide if there is, in their opinions, enough evidence to proceed to trial. I choose not to comment on you calling it a "farce."
There is no way for either of us to know what Hunter's reasoning was but the fact remains that a grand jury heard from both sides, pro and con, on available evidence and believed there was enough to take the Ramseys to trial.
A DA knows no such thing in advance about whether or not a trial jury will find a defendant guilty or not guilty so your statement is, at best, misleading.
Unsourced DNA is neutral. If or when it is sourced it might be useful. Until then, it means nothing one way or the other.
Ramsey DNA was found on items as well and IDIs often want to excuse that as meaningless. Depending on location and probabilities, touch DNA from a family member may mean nothing or something.
Deciding guilt or innocence based on only one piece of evidence is ludicrous.