Can you state what those reasons are. We know the police were responsible for staging Sheila's body in the right photograph. You can see that the police moved the body by about six inches.
Who's to say that both photographs don't show Sheila's body as it was staged by the police?
Given they were messing around like that, accidentally shooting Sheila when she was already dead is a realistic possibility.

According to the description given in the Telegraph,Sheila's hand had been " curled " around the barrel ?
When one has been dead for several hours,the extremities ( fingers and toes ) are the first to go into RM,along with the neck/chin/eyelids and discolouration begins in minutes.
If Sheila had been dead for some time,it would have been difficult to have unfurled her fingers from around the rifle,that they'd have been left in a " claw-like " position.
This isn't evident on the pics which means that she hadn't been dead above 2/3 hours for those involved to have left her in the position that we see. I bet she was even still warm if they'd have tested her belly.