Not understanding their motives *spoilers*

I think I may be able to help a bit. My feeling is that the reason Jenny is "so perfect" is that she strongly resembles the first victim from decades earlier. (The vicitim's ghost is also played by Laurie Walters.) Not sure if both cops were in on it, but the younger one certainly appears to be. He's the one who "finds" the marijuana during the end credits, and earlier the old woman and/or John references "Luther and the others" (meaning other cult memebers.) Luther is the younger cop, although I'm not sure if he is called by name except near the beginning of the film when the Jenny and John get pulled over. (That scene is in the VHS release but not the DVD.)