It occurs to me that we are all attempting to create THE scenario from numerous diverse theories, some of which will be rendered invalid.
Starting from the point of Jeremy being innocent, I would be interested to know which scenario is favoured by the majority and is it plausible.
Logically, I can see reason for going with Mike's theory of it being a police bullet which finally killed Sheila, giving rise to the need, at all costs, for a cover up and a scapegoat. Emotionally, I favour the presence of a third party with a grievance against the Bambers, because I want to exonerate Sheila.