I'm not saying it is impossible that Sheila would escape being splattered with blood, or that her fingerprints wouldn't be found in abundance on the rifle and her hands revealing a low count of GSR, however, all three and no othe physical evidence is just a stretch too far. Sometimes it's what isn't found that's important.
All the science underpinning these aspects of the case suggest SC's found state is entirely consistent with her as perp:
The .22 long rifle and sub-sonic ammo wouldn't necessarily produce back spatter capable of reaching the perp. The blood stains on the rifle don't resemble high impact back spatter. I understand victims' physiology changes with each subsequent injury lessening the potential for back spatter. There's no evidence NB's non gsw's caused an abundance of blood. Had they done they would be all over the place eg ceiling, furniture etc. It seems blood stains on the kitchen worktop and a tile near the Aga were caused when NB put his blood stained hands out.
Firearms rarely yield fingerprints for the reasons contained in the link I posted the other day.
GSR was useless in this case since whether SC victim or perp she was found in an environment where the rifle had been discharged numerous times and twice over her body. DC Hammersley soc officer responsible for taking the swabs for GSR testing, was like DC Bird a novice and, made numerous mistakes rendering the test/results meaningless in any event.