Part I don't get? What body did they roll up?

I'm not sure if you had seen through the end. I mean there's an answer of how Sam and Suzie set up the blood and teeth evidence on the boat after "The End" title. Sam pulled one of Suzie's teeth, and she pulled another herself. That explains where the blood stain came from. For the body, it was really Suzie wrapped up in the car as there was a close-up shot of her face. Sam might have not wrapped her up too tight, leaving some captivity for her to breathe for a while. Also he was the only one who took care of the body disposal, which actually was letting Suzie out of the bag and going away hiding some place after their plan. This happened somewhere in the swamp where Kelly didn't witness (she was waiting on the bank) and never had a chance to know it. Therefore, no body was switched. Just a fake murder to deceive Kelly.

As of the body to prove Suzie's death, you can tell from the scene when Ray was called for his career termination when the committee said about the blood and teeth on the boat and Suzie's car left out somewhere near the bus stop, and then made a conclusion that she was murdered and already dead. So we might assume that Suzie's body was never found, neither was any anonymous body as a fake evidence. At first I also doubted it. Why such a quick case close with such a few evidence ? Then I thought it was for the suspense's sake to leave it uncleared and fulfill the end twist that Suzie was still alive.

This seems to be the only huge plot hole in the movie. When Suzie's body is loaded into the back of the range rover it looks like it's wrapped super tight. There is no way that Sam had enough time to poke holes that were inconspicuous and would also provide enough air for a girl who is probably choking on her own missing tooth blood. I think this movie is a rare gem in that it's a great mystery wrapped in enticing smut but this one part with the plastic has always bothered me.