But those that 'discovered' this evidence stood to inherit a two million dollars worth of estate If a conviction was successful. Its not reasonable to have 100% confidence in this.
On the contrary it is unreasonable to reject the evidence on the basis you do. The family didn't know a thing about drawback, didn't know Sheila's fatal wound was a contact wound let alone a contact wound that would result in drawback, didn't know her blood type, didn't have access to her blood and even if they wanted to plant blood for simplistic reasons without appreciating anything about drawback they would not have used something to spray blood inside. Yet the blood inside the moderator was sprayed inside, that is the only way for blood to get on the first 8 baffles. Furthermore, if the family had planted the blood then Sheila's blood would have been found in the muzzle of the rifle and the family had no access to the murder weapon so can't have removed such blood. If they had known her fatal wound was one that would result in drawback and had known all about drawback they would have known her blood was in the rifle so planting blood in the moderator would be useless.
In order to reject the evidence on the basis of the family finding it you need to establish there is a reasonable chance they planted the evidence. There is not a reasonable chance they planted the evidence. The evidence is safe.
The family are part of the chain of custody. The house was in police control until August 9. August 10 the family, Basil Cock etc went to the house and the moderator, bullets and various weapons were found. The family established the chain of custody of all such weapons. They only had the moderator for less than 2 days, keeping it in a closet or chest until turning it over to police on the 12th. From there the chain was in police custody or at the lab depending on the date. This is sufficient for a court to rule it reliable evidence.
To reject it you have to establish a reasonable likelihood of planting the evidence in question but the defense could not do so the defense had zero basis to suggest they planted any evidence.