Does anyone here disagree that the scratches on the mantelpiece were not there just after the murders?
If you do, why?
Your question is ambiguous. It is unclear what you are driving at.
The scratches were definitely made by the suppressor. There are only 3 possibilities of how this occurred.
1) The scratches were made before the murders
2) The scratches were made during the course of the murders
3) The scratches were intentionally made after the murders
Which is most likely?
The scratches are not linear, they zig zag around. If the gun fell on the floor and someone simply picked it up and accidentally scratched it at the weapon was raised then there would be a mainly linear scratch.
If someone intended to intentionally scratch it they would tend to move in a linaear fashion.
The scratch is consistent with two people moving it back and forth as they fight over control of it. It just so happens that during the murders Nevill and his killer struggled over control of the weapon by this very mantle.
If not for this struggle then the killer would simply have shot Nevill to death and would not have needed to bludgeon him with the rifle. But since he was able to grab the rifle the killer first needed to regain sole control and needed to get far enough away to raise the rifle and aim it at Nevill.
The men wrestled over the gun breaking things in the process and knocking them over. Their bodies knocked over chairs and the like but the gun crashed into the ceiling light fixture and knocked other things on the floor. The killer punched Nevill breaking his nose and giving him black eyes and the killer eventually wrestled the gun away then bludgeoned him with it. The killer attacked his right side. Nevill blocked the blows with his right arm but the killer eventually got him in the head knocking him out, breaking the rifle stock in the process. The killer was then able to aim the weapon into his head as he lay passed out
There are no known struggles over the gun prior to the murders so it is most likely the scratches occurred during the murders not in a prior altercation. Indeed the ceiling lampshade being broken is consistent with the suppressor being attached during the altercation (the rifle probably would not have been high enough to break it otherwise) plus the fact it was used during the shootings further suggests it was attached during the struggle.
Heck the defense at trial suggested it was Nevill and June's blood in the suppressor. What is the likelihood the killer would remove the suppressor before running into the kitchen?
The chance of the marks being made before the murders is too remote to be seriously considered. The chance of occurring during the struggle? Very probable. The chance after? Unlikely to be staged at all let alone correctly staged instead of staging linear scratches and the motivation for staging is specious.
All in all the evidence points to the scratches being made during the known struggle over the weapon.