What bothers me is that there is absolutely no sign of Sheila having moved between the two shots. I don't know how much time there was between shots, but if Jeremy did it wouldn't he wait a bit after the first shot to see if she died? Surely her instinct would have been to put her hands to her throat after the first shot, but there's no sign that she did that. Alternatively, surely she would have put her hands on the gun to push it away, but there were no fingerprints to suggest she did that. He might have wiped them off I suppose.