The moderator could have been used and Sheila could have placed it back in the cupboard before she used on herself. This could explain why June's DNA was found is the moderator. Who is to say what happened, or how it happened, its case of what you believe in and what others might believe in. 
June had no contact wounds let alone contact wounds that were in areas that would result in drawback. June's DNA was not blood based, the blood already was removed years earlier no blood was present when it was DNA tested. June's DNA was even found in areas of the moderator where no blood had even been found in 1985 and 1986, indeed the areas were too deep for drawback to reach such areas. Furthermore, there was no way her blood could have mixed with Nevill's into the flake and yet for the flake to not have her Ak2-1 enzyme and not to have enabled the lab to know it was a mixture. The defense experts admitted that if the blood intimately mixed it is impossible for the lab to have made any such mistake and that the only way it was even possible to miss a mixture would have been if the blood failed to intimately mix but they failed to establish it would be possible for blood of 2 victims not to intimately mix in the moderator.
In any event even if June had suffered from contact wounds and her blood was found inside as well as Sheila's it still would not have helped matters nor would finding her blood inside help. The notion SHeila went crazy then went to the close to get the moderator to attach to the weapon Jeremy left out so she could silently kill everyone in their sleep then to take the moderator and put it away in the closet before killing herself is not in the least bit credible. The only way it is credible she would have used the weapon with the moderator attached is if it were attached already when she picked it up. Upon finding it inhibited her shooting herself she would not remove it and go downstairs to put it away she would just leave it in the room where she was trying to kill herself.
The moderator used period harms the claim she did it given the moderator was put away and given Jeremy claimed it wasn't attached when he left the gun out.
If he had had removed the moderator before shooting her and simply left it in the bed room and made her lie flat and had the gun across her body as she would have done to shoot herself in the chin he would have eliminated most of his problems.
1) that would eliminate the moderator problems and result in drawback in the weapon
2) it would result in soot getting on her gown
3) would eliminate having to move her body flat later which gave away she was moved by someone after her death
This assumes he could have gotten her to lie flat maybe he could not have gotten her to do it who knows. If he could not get her to lie flat then he should have just left the gun near her or on her legs as opposed to moving her body that is a no no.