It's not that prosecutors are unsupervised, it's that a grand jury is only hearing from the prosecutor's office; neither the defendant nor their lawyer are allowed to be part of those proceedings. The DA presents its case in order to get the jury to agree that felony charges can be filed.
The DA may be tempted to bring a lopsided case to the grand jury, if they feel that a felony indictment will make them more likely to plead to a lesser charge. If the case seems weak, they may push for a felony indictment even though they plan to drop the charges if the defendant won't agree to a deal. If the case has made headlines and the public expects action, they may be forced to indict regardless of the probable outcome.
I think this case has the latter as one of the factors. Complicity in a crime that is so horrifying isn't something they can wave off, though they might be using the indictment as a way to pressure her to cooperate against the other defendant, who appears to have perpetrated the actual assaults.