I am not sure of the DA's responsibilities in a grand jury setting -- his job is to present evidence a crime took place, and by whom, and not to develop a defense. But in this case, if the DA had been presented with alibi evidence and ignored it, he might have a problem. It's one thing to present a clear prima facie case; it's another to ignore exculpatory evidence.
As much as I have read of NC law, it appears that the prosecutor has pretty much free reign. He can call whatever witnesses wants, or not, and show or not show any evidence. For example, I believe it is entirely possible that the grand jury didn't know her injuries happened before the alleged incident, or that the DNA tests came back negative. He just has to hope no jurors get smart and ask to hear certain witnesses they think would help.