I don't think the person in the video was alone. I think what they were doing was performing for the cameras then went to the room where Missy was killed and there was someone else in there who actually killed Missy. But at this point, it's pretty obvious that this case will not be solved...
I also think the killers of Terri Bevers not only attempted misdirection by faking the church burglar act, but they very likely also created misdirection by using the video cameras to make it seem like only one person was in the church before Terri arrived. The video is a lie, from a sociopath...