I'd like to know SS's full reasoning for coming to that belief. It seemed to me that it was only because he saw no evidence to indicate gun shot wound(s) to the head, or blunt force injury, via blood spatter at the home, or whatever. But yet according to the autopsy report there was no mention of GSR being found.
If there was a small bit on Mr.D's shirt, it could have been from taking a shot himself at some point (wild rodent or something?, perhaps not during that particular day even), or even 'transfer' from the killer's hands or clothing, and nothing to do with this particular crime. I haven't heard whether the Ds owned a gun(s) or not, and if so, if it/they had all been accounted for? It doesn't make any sense to take someone and then bring them back to decapitate them. Not that this murder makes any sense to begin with, but that seems really out there, to lug him back to his garage, already deceased, only to then decapitate, exposing themselves to more risk of getting caught.
When they discovered Mrs.D's body in the lake, did they do any research on where it is believed she would have originally been sunk? ie going by the motion of the water, in relation to weather/wind conditions during the 10 days until she was found, etc? It would have been useful to have an idea where she had been dropped into the water, and then perhaps to have searched/dredged that surrounding area to see if Mr.D's head might be at the bottom attached to yet another concrete block?