I don't think this was an accident at all. Here is my theory:

I think someone was stalking and harassing Teresa. They knew her schedule or somehow found out that she would be visiting a formerly convicted felon that day who would make a perfect scapegoat. They knew how the police felt about the Avery family and knew any investigation would center around Stephen. They waited until she left the Avery residence, then abducted her. If she knew the person they may have called her phone and asked her to meet them, which could be why they deleted the voicemail. After doing whatever they wanted with her, they killed her and disposed of her body in the woods near her car.

This is where the police come in. While they are searching for Teresa, they find her car and burned remains. That is why the one officer calls in asking about her plates. They already have Stephen in custody and want to pin it on him, so they know they must make this evidence work for the case. They take both the car and remains and plant them on Stephen's property so they have evidence to convict the man they already believe killed her.

Summary: the person who was stalking and harassing Teresa killed her for whatever sick reason and used Stephen as a coincidentally perfect fall guy. The police planted evidence either because they believed Stephen was the killer and wanted to make sure he was put away for good, or to protect themselves from his pending lawsuit. Everyone else went along with the lies or half-truths for their own personal reasons.

I believe Teresa's brother and ex-boyfriend may have both been stalking her and did this horrible thing to her together. Her brother still seemed pretty friendly with a guy who was her EX boyfriend. That coupled with his lack of emotion when talking about Teresa, and her ex-boyfriend's reputation for stalking... It would be very strange if this were all true, but strangers things have happened.