@Adam
For now, my interest is in the plausibility of Sheila as a basis for overcoming the convictions separate to attacking the silencer evidence.
The major points against Sheila as the killer seem to be [I don't pretend this is exhaustive and ignore certain evidence like the Bible as I think the police messed around with the crime scene]:
1. Sheila had no meaningful experience with guns.
2. Sheila was not normally violent against people.
3. Sheila was uncoordinated or her motor skills were impaired or she was drowsy or totally sedated, or some variation on this theme, hence it's unlikely she did it.
4. A suicide shot would normally be intra-oral, but her two gunshot wounds are to the neck-throat region, suggesting staging. It was difficult enough for Jeremy to shoot her as it is, but much easier to shoot her in the neck/throat than intra-orally. Conversely, it is easier for Sheila to shoot herself intra-orally than in the neck/throat. Therefore, the wound locations point to Jeremy as the killer.
5. She had only very small traces of incriminating residues on her that could have got there through normal activities.
6. Nothing incriminating was found under her nails, despite a belief among the SOCO team at the time that she had committed suicide after struggling with Nevill, then killing him and the rest.
7. Only one recorded fingerprint on the rifle, despite a belief among the SOCO team at the time that she had committed suicide and killed the rest by using the rifle, including maybe two or three reloads.
Probably there are more points you can come up with, but that'll do us for now.
Can these points be assailed? Points 2 and 3 are of most interest because there is scope for fresh evidence on these.
I don't believe Jeremy can overcome point 4. The only reply I can think of is that maybe Sheila wanted to preserve her face and perhaps she had a mistaken belief that she could do so by shooting herself in the neck/throat rather than in the mouth, or maybe she didn't know how to kill herself by shooting in the mouth, but I'm not very convinced. Sorry to be morbid.
I think he can overcome 1, 5, 6 and 7. Points 1 and 7 can be waved away. Points 5 and 6, the problem of a lack of residue and traces under her nails, is easily addressed through Professor Knight's theory. If she cleaned herself, it would remove the forensic footprint, not totally, but to an extent that it might not be recorded by the police. As bizarre as it may sound, it is plausible that she would clean herself. Whether she really did or not doesn't matter for our purposes. The point is that it is plausible.
That just leaves point 2 and 3, which as I say are the ones where there is scope for fresh evidence.
On point 2, if expert evidence was forthcoming that confirmed she potentially could have had a violent psychotic episode of that magnitude, that would quash the conviction, though it probably wouldn't be sufficient for an acquittal should the Crown proceed to re-try the case.
Point 3 is the stronger of the two. If it can be shown that the sedation explanation for Sheila's actions is misconceived and she could have been alert, then I think there's reasonable doubt.