Though I agree that it's difficult to prove a murder without a body, wasn't there a recent case in the area of a young woman disappearing and there was a conviction, murder one even I believe. Alexis Murphy case. Not familiar with the details, but I do recall that she was not found. Famous case of Lacy Peterson proceded without a body, though it did eventually surface with a conviction for her husband Scott Peterson.
I really think the key is the car. Longo seems confident that forensics is going to come up with something from the car, and I get the feeling that he thinks it's going to be something big. There is mention, I found that the car shows up on some footage, but it's never shown, from what I could see, and not much info given as to what LE saw. Something they saw has them hot about that car. I think the car led them to JM instead of the video and info on JM to the car.