It am just pointing out that broken bones in a persons neck would cause swelling and constriction. Sheila would have difficulty putting her head back once her vertebrae were shattered cannot see how Jeremy could have forced her head back to shoot her on the floor, more possible if she was sitting on the edge of the bed however even more likely she shot herself in this position. imo.
Swelling is not instant, swelling progresses over time, it would not be unmovable in a matter of seconds.
If she had been on the bed she would have fallen face first onto the floor or backwards onto her back. She had to be leaning against something that prevented her from falling back to stay seated. While Jeremy could have prevented her from falling foward he would not have prevented her from falling back unless he immediately grabbed her to hold her her up after he shot her. Why would he do so?
Shooting her lying down would have made life easier for him but in that case the blood would not have been all over her shoulder or the 5 drips of blood land down her arm. She needed to be seated for several seconds immediately after the first shot for that to happen.
The chair in theory would work but no evidence was developed that suggests she had been in the chair such as blood that dripped on part of it to reveal she had been in it. If she had been in it only a short while maybe then there would not be much blood so it would be missed but there is no way to say it definitely happened.
Moreover, it would be more tricky to lift her from the chair onto the floor. Lifting her up runs the risk of blood dripping on her gown so that it would have appeared she had been standing at one point.
Having her sit on the floor would be easier especially if he wanted her to look like that is how she killed herself.
He probably realized afterwards that seated the gun could not sit on top so that is why he moved the blody flat.
Is it possible she sat for as many as 10 seconds then he pulled her body flat and immediately shot her while her body with her body flat on the floor? In theory it is possible.
The main thing is that she was seated against something while the first shot was being delivered, sat upright for several seconds after that shot then either was shot again while still seated or pulled flat then immediately shot the second time all in the space of around 10 or less because otherwise she would have substantially more blood on her. The whole reaosn we know which sot was delivered firs tis because the hemmoraging from the wound. Had she been dead when the lower shot was delivered she would have bled substantially less. Had the gap between the shots been longer she would have bled more because she was bleeding substantially.
If one wants to try thinking extra hard then they can suggest:
1) the loose pieces of carpet were used to put the chair on so that it would not leave an imprint on the rug (extra carpeting like this is routinely placed under the feet of exercise equipment and other things that leave indentations in carpet and one doesn't want indentations in the middle of the carpet; it is also to a lesser extent is used to prevent chairs from leaving indentations though more often a plastic protector will be used for office chairs).
2) chair placed in front of the nightstand next to the bed on the loose carpeting. Sheila forced to sit in chair. She is then shot in the chair.
3) She is shot in the chair a second time and then the chair is lifted out from under her very carefully and she is placed flat on the floor or it is lifted out carefully and she is placed flat on the floor after the first shot and the second shot is delivered as she is flat on the ground already.
A normal person would try to kill her in the position they want her to be found in or as close as possible not wanting to bother having to take her out of a chair and risking getting blood smudged all over. So if the plan was to leave her flat all along then the extra work involved in shooting her in a chair is not likely. It would be easier to just plan to drag her flat. But the goal might simply have bene to shoot her seated on the floor with the gun nearby, it realized it was hard to fit the gun close with her seated so to pull her flat to stick it in her.
