I can see what you are saying about the blood flow under her arm Lookout .
I find it hard to imagine either scenario really
Jeremy crouching holding the gun in both hands and getting at the right angle to make it look like suicide but without her fighting back or moving or him disturbing anything in that small cluttered room ?
Or how she held that long rifle to kill herself . OK its proven she could do it with her foot but would she know that ? could she have done it without using her foot without the silencer on?
Firstly,there was no doubt in my mind that there were struggles going on between the three adults,mum,dad and Sheila. All three had fingernail gouges on them,obviously by trying to retrieve the rifle from Sheila. IF Jeremy had been there too,he'd have had them,no mistaking that as he'd have been hard-pushed against the manic strength of his sister.
Sheila would have found a way of shooting herself,but certainly not with her toe on the trigger. The girl had a long reach anyway being 5.8 so her arms length would have been in keeping/proportion with her overall body size.
I believe she was in a sitting position when this was" accomplished",as the bullet went upwards. Her head then bent which would have stemmed the flow of blood,and the pressure would also have stopped due to vascular shock and that's why the blood had pooled in the crook of her neck. Because she was against the cabinet,she'd have maintained that sitting position until such times as she'd been placed where we see her.
Those darker spots on her arm,as I've said,would have dropped from the neck wound ( jugular ) while in the sitting position. It's the only way they'd have got there otherwise. If she'd been lying down when she shot herself,those spots wouldn't have been there.
Nobody but herself could have shot her in that position. With two shots,you can NEVER make it look like suicide------Jeremy would have known that as it's common sense,a child would realise it. The fact that the rifle was semi-automatic and comparatively new would make it sensitive to a second firing to those who weren't accustomed to using them and because the shots to her neck were pretty close together,it would make this possible.
No way do I believe that Jeremy had any part in this,nor did he know or even realise it could happen.