You may be right but I am not convinced the fingerprint evidence takes the case forward in either direction.
Isn't the problem that if Sheila was the killer, then when killing herself she must have got quite a tight grip of the rifle? This is especially the case when you consider her slight frame and the angles at which she supposedly shot herself, twice. That being the case, shouldn't there be more than one single fingerprint on the gun from Sheila, if she did it?
I am willing to accept that Sheila herself could have wiped/cleaned the rifle before killing herself, and this may explain the smudging. Let's allow that, and let's also remember that Ron Cook handled the rifle without gloves. Even so, wouldn't she have still left more recordable prints?
On the other hand, if Jeremy is the killer, then we may only see one single print from Sheila because Sheila has not put any manual pressure on the rifle, rather Jeremy has positioned the rifle in her hands, probably after shooting her at least once, and he has perhaps deliberately attempted to make prints from her fingers, and succeeding in establishing one. Gloves may also have caused the smudging of prints resultant from normal handling of the rifle prior to the incident.
Do we have a diagrammatic representation, or even photographs, of where the smudged prints are and the pattern formed?