Surely to God a controlled experiment could be carried out to prove things.
All you need to do is attach a tube with a valve on it to a volunteers neck,then instruct them to carry out the accepted series of events concerning Sheilas death.
Someone would open the valve and let the artificial blood[Or real blood for that matter]flow at the precise time the shots are re-enacted.
If the blood ends up in a similar fashion on the volunteers right arm,so be it.If the experiment does not produce this with repeated attempts and amounts of blood flow,then it would be assumed the blood was present from other wounds prior to Sheila being shot.
Possible?