The bewildering question that stands out in my mind is why would the killer take the risk of removing a body from the crime scene rather than simply leaving it?
Is the simple answer that SD had been the target for sexual assault?
Assaults on elderly women are not unheard of. If this was the case, it becomes much more understandable that the killer had no option but to get rid of the body because of the DNA signature.
It's also possible that the killer was unaware of the fact that a corpse will float after being in the water for some time. He may have been so unaware of that, that if he simply dropped the body in the water and it quickly sank out of sight, he thought that was it. He could have weighted the body with almost anything he could pick up, and thought that was how it was done in the movies. He may never have heard of Archimedes.
It may be possible that a boat was used, but it's not absolute---I don't think. If the killer was a "regular" in GW, he could have driven his often seen vehicle to the end of the cul-de-sac, out of sight of the neighbors whom may have been otherwise occupied, and made his move on SD. RD could have been off on his walk, playing golf, or at the Publix chatting with the cashier. If the killer knew RD's normal schedule, that would have made the attack even more convenient.
So the killer may have planned ahead or realized after that he had to get rid of the body. Given the house being located at the end of a cul-de-sac, it's likely no one would come up, drive around, and drive back out, and pay any attention to what might be going on. Especially if the vehicle's was familiar in GW.
I would think it would be safer and easier to remove the corpse from the home, place it in the vehicle, and proceed to the point of placement rather than do something like pick up the corpse, walk around the house down to the little dock, put the corpse in a boat, and leave in the boat with a dead body going out onto the lake where there would potentially be more witnesses, and the chance of doom if someone saw something odd on the cruise to Riley Shoals.
Was it day or night? SD was, according to LE, wearing day clothing, so it seems this was a daytime crime?
So what about RD? If he was out and came back at the time of the crime, the killer had to eliminate him too. What happened then is another mystery, but the finish was the same. If RD was simply killed, or even as he was killed, there's little likelihood of a DNA signature, so it wasn't mandatory that his body be disposed of. Why only the head? That's a tough one.
If the killer used a vehicle to quietly leave the scene via a route that was nearly unobserved, (familiar vehicle through the gate), then there is a route to, possibly, Long Shoals, possibly somewhere else, and that route is not busy with traffic or residential development. But is it possible that he waited until nightfall to dispose of the remains at a place a short distance from the place where the corpse was when it was found on Friday? Could the corpse have been placed in the water such that it could have, after some days, have come up at the snags?
Hmmmm.....