There was a lot of stuff in that swamp, and with a tropical storm through the area, it could have been swept there. Not only that, but people have done weirder things than make accidents look like murders. Furthermore, the only person the duct tape leads to is George. Even more, Roy Kronk said a lot of different things. The white board over the site, the tree over the spot, the skull in the open, then in a bag, and the four month period of waiting to recall it in. Obviously, with his different stories, the scene changed. That makes the whole scene tainted, so we don't know if the duct tape was there in the first place. As to the lying, why would they own up to anything? You said it yourself; they lied throughout. George wouldn't even own up to the affair. With all that, and no evidence that it was even a murder, there is reasonable doubt.