You are incorrect. Whether it is a grand jury or a district magistrate that determines whether there is evidence to proceed with the trial it is the prosecutor that presents such evidence, and the goal, anyway, of the prosecutor is (supposedly) not to get a conviction but to see justice done.
IOW, if the prosecutor should come across information -- and the prosecutor should be open to such information -- the prosecutor has an obligation to stop the trial.
Apparently, in this case neither the prosecutor nor the investigators even bothered considering DNA evidence.
The grand jury has the ability to deny a prosecutor the indictment if they decide that, and grand juries have done that, many times in many locations. It happens all the time.
Furthermore, the grand jury can order more information be sought, for example, if they didn’t see that any DNA evidence was gathered and/or sought, they could order it themselves, if they wanted to. The grand jury has powers to do that sort of thing. Just because a certain grand jury of citizens decided to not go any further than what he did — that’s their problem and their responsibility. They didn’t question things any further, themselves.
And once an indictment is handed down, it’s not the job of the prosecution to be developing evidence to help in the acquittal of the accused, but to get a conviction. That’s their job. And if they do come across evidence that clearly makes it impossible for the accused to have committed the crime, then they must make that available to the defense. The defense would be the ones to bring that up to get it dismissed at that point.
Once the trial is going, it’s the job of the prosecution to get the conviction, with all his abilities and it’s the job of the defense to get the acquittal.
And then, it’s the job of the jury to determine what is final decision. The responsibility lies in the jury’s hands, and if they think something is left out or not considered and that this possible evidence would be critical in deciding the case, but it was never presented, then the jury can say that this introduces too much doubt in the case and declare the accused not guilty on that basis (their doubt as to the case being “proven”).
It’s the responsibility that is in the hands of the jury.