What the family did and did not know cannot be fully corroborated. However looking and the evidence it cannot be ruled out 100%
It doesn't have to be ruled out 100%. It just has to be ruled out as being reasonably unlikely. You keep ignoring the actual burden because you can't meet it.
You need to establish it is reasonably likely that the blood was planted by the family or police and that it is reasonably likely blood had been found in the rifle but police or the lab cleaned out the weapon and concealed the finding of such blood. If you can't do this then there is no reasonable basis to doubt the blood evidence.
Don't jump to a conclusion but read all I have provided below....
Malcom Fletcher admits the weapon in question is the least likely to produce backspatter.

No what he said is that GENERALLY SPEAKING .22lr bullets are less likely to cause backspatter than larger caliber bullets. He said however an assessment of backspatter requires looking at the exact circumstances not making broad generalizations. The location of the wound is of the most importance.
The wound location was full of blood, she had already been shot and her neck was full of blood because she had hemorrhaged. The nature of blood and skin in that area is already good for drawback to occur in the case of a contact wound but because of the area was full of blood it was certain to occur.
You and others choose to ignore the realities. The reality is that neither at trial nor on appeal has the defense found ANY expert who refutes that drawback would be certain to occur under such conditions. Not even Webster claimed such. That was one of the reasons why the Court of Appeals said he failed to undermine the prosecution's case he failed to address let alone refute that had the moderator not been attached then blood would have been found in the rifle.
That drawback is even mor elikely when a bullet is larger than a 22lr doesn't render drawback unlikely when a weapon used is 22LR. You have to make assessments based on the location of the wound. Drawback is UNCOMMON when a 22LR is used for a shot to the head but even in those cases there are instances where it happens but it doesn't happen a majoiry of the time. So if that had been the case the defense could safely have argued that drawback would not necessarily occur. But they had to deal with the facts of the case which was a 22LR shot to a body area full of blood and thus that would result in drawback. The only way to try refuting drawback would occur in such location would be to try to find evidence to prove it wasn't a contact shot. If not a contact shot then blood would not get much more than 5mm inside. The blood was found up to 40mm inside. They found no such evidence though.
The only other possibility he sais is deliberate planting of evidence, If he is under the impression it is Shelia's blood then its understandable that he can produce that unlikely alternative.

Fletcher then in 2002 agrees with you saying that Shelia's blood would be in the barrel of the weapon had the silencer not been attached. But he does not rule it out 100% saying there is a slight possibility of it not happening.

His comment was meant to rule out innocent contamination. He was effectively saying it is impossible for the blood to have gotten there through innocent contamination the only way blood could have gotten in the distribution found was drawback or deliberate planting. He wasn't asked what it would take to deliberately plant it though. It would have required spraying it inside using some sort of device.
A very slight possibility is not good enough. The defense needed to establish BOTH of the following it was reasonably likely that blood would not get inside the rifle (very slight possibility doesn't cut it) [or in the alternative that it was reasonably likely blood had been found in the rifle but such finding was concealed] AND that it is reasonably likely the blood was planted in the moderator. You need to establish both in order to refute the prosecution's evidence.
Maybe because I am a lawyer I recognize what the court said is required, maybe because I like guns so much I am able to understand it, maybe because I am not biased I am able to face it while others biased in favor of Jeremy close their eyes to it. The reason why I recognize what the defense needs to establish is not relevant the fact of the matter is that I do and have explained it.
If the defense doesn't face what it needs to establish to get his conviction vacated then the defense is never going to be able to do so because the first step is recognizing it and then seeking evidence to prove it.
How to point of the Blood in the silencer, There is a substantial amount criticism of Mr Hayward's work and his methods of how he produced his results that seems very overlooked. By including
A) Dr Patrick Lincoln came to some different conclusions in his results than that of Mr Hayward.
B) Mr Hayward on document (wrongly dated) probably typed by error 3rd of August instead of 23rd sais the possibility of a mix of June and Neville's DNA is a possibility, This is also considered possible by Mark Webster

Now most damning for Mr Hayward is that he admits his decision was 'influenced' by the Autopsy report of Vanezis. Going by Vanezis claim of a contact wound let him to believe it was more likely Shelia's blood and not a mix of the parents blood. What ever your position on the case it I don't think this is very competent! You should but the samples under 100% of your own professional means without any outside influence specially when it comes to DNA and forensics

First of all you ignore numerous things.
1) the defense failed to establish there is any way that blood of 2 people shot around the same time could fail to intimately mix. This was fatal to the contentions of the defense at the Appeal hearing. They could not establish it scientifically possible.
2) You are ignoring that even the defense admitted it would be extremely unlikely for anyone to be able to miss that there was a mixture if blood had intimately mixed.
3) The reason why the autopsy is significant is because Neither June nor Nevill suffered any contact wounds let alone contact wounds that were likely to result in drawback so their blood could not get more than 5mm inside. Blood was found to the depth of 40mm and Sheila did have a contact woudn that would result in drawback.
The combination of this makes it clear it was Sheila's blood. It is not reaosnably likely the blood was a mixture of June and Nevill's blood.
Now putting all the above together it does open the possibility of
A) Manufactured and falsified evidence in the sound moderator
or
B) Shelia shooting the entire family resulting in Neville and Junes blood getting in the moderator then putting the silencer away then shooting herself later.
So I think its safe for me to say Jeremy's innocence is Possible but Very Very Very Improbable and If you rule out the extended family framing him, An Innocent Jeremy is more unlucky than someone being struck by lighting twice then being knocked out by a falling coconut.
You got all the issues wrong and worse you failed to do any inquiry related to the family's ability to plant blood. Establishing it was reasonably likely they planted it requires proving they had the requisite knowledge and ability (including access to blood of the victims) not just they had the item in their possession. If they had tried to plant blood it would have simply fallen inside the opening. They would not have sprayed it inside which is the only way it could have gotten 40mm inside on 8 different baffles. In addition it would be necessary to establish police either found blood in the rifle and concealed it or Sheila's wound wasn't really a contact wound.
Based on the existing evidence there is no way to establish it is reasonably likely the blood was planted by anyone. The only people with the requisite skill would have been the lab. Likewise they would be the ones who would have been in a position to have concealed the finding of blood in the rifle. So someone who in earnest was trying to establish doctoring of evidence would try looking there.
This doesn't even take into account the paint. The police would have to have planted the paint and despite this the lab decided to plant blood. In the meantime the family and police did see some blood inside the opening and on the outside face. So the moderator was used and the contention of planting would have to be regarding the blood found deeper inside which was a sign of drawback.
So essentially Sheila had to have used it to kill the others, put it away before killing herself (which makes zero sense) and then the lab planted more blood to make it appear there was drawback inside but if they had planted it then they would have made sure they found Ak1 in all the samples not just the flake.
The allegation Sheila went to the closet got the moderator out and attached it then used it to kill everyone else then put it away before killing herself is all the defense had and has but it is not credible and not reasonably likely. So on top of the fact they can't refute the other evidence the story they try to say happened still fails miserably.
One has to be biased in favor of Jeremy and thus to ignore the evidence or not comprehend the evidence/applicable law in order to argue Jeremy wasn't proven guilty beyond a reasonable doubt.
Such arguments will not sway people who actually face the evidence and comprehend it and the law. That means they have no hope in hell of convincing an appeal court unless they can establish the existing facts are wrong. They have no way to do that though.