I remain convinced that a ring on her finger -a VERY big one, mind- would have made a vast difference.
Julie seemed genuinely bothered by how Jeremy was acting after the murders and said part of why they broke up was she wanted him to confess. We have no way to know whether a ring would have caused such to subside or not. Maybe it simply would have caused a postponement of when she ratted him out. Their relationship would have been rocky even with a ring so she would have likely told her friends anyway which is what got the police involved. It's doubtful their marriage would have lasted even if they had gone through with it.
Even without her evidence they still would have found the blood in the moderator her claims did not lead to such.
At that point they could have decided to go after Julie even if her friends had not said a thing just to see if they could rattle her and see if she knew more.
That they were broken up when questioned eliminated part of a dilemma she would face. Do you protect the person in order to protect yourself because you are totally invested in spending your life with a killer or do you decide to give up the idea of a life with a killer and decide to let the person go to jail? If they were still together she would have to decide. That decision is a little easier when the police have other evidence against the person. It is already out of your control in a way.
If broken up already this decision doesn't come into play it is not a matter of protecting the person to protect your self. You just decide whether to protect them for the sake of protecting them.