Just because the hand swab results don't match your theory, does not make them worthless. In fact they are compelling as the lab were concerned with contamination. If the result had been positive then indeed they would be worthless, but as they were negative, provided very powerful evidence that Sheila did not fire any rifle.
Maybe you could answer my question?
How is it possible for Sheila to be presumed dead on the kitchen floor, make her way to the bedroom, shoot herself, be presumed dead again, only to be shot again sometime later?