I'd like to revisit the issue of the blood on Sheila's arm. I think these are runs from drips of blood and not the result of injuries to her arm. That would suggest that she was standing up or at least sitting up when she was shot and that her right arm was directly below her chin at the time of one of the shots so that the blood dripped onto her arm. That would probably be the first shot.
If she shot herself and then fell backwards, it's not much of a mystery, but if Jeremy shot her, he had to make sure that he positioned the rifle correctly, and I'm not sure how he would have done that. Did he ask her to stand or sit still whilst he shoved a rifle up against her neck? Perhaps he did just that and she had her hand on the rifle trying to push it away, which is how the blood dripped onto her arm.
I do think that the evidence shows that she was shot where she was found - I see no evidence that she shot herself once and then walked around.