Two questions, kind of in opposition to each other.
1. Is it possible that if it was JB who carried out the crime, then the suicide staging simply presented itself as an opportunity, rather than it being a premeditated plan?
2. Even if the two bodies downstairs turned out to be nothing more than a communications mix up, and Sheila was already deceased when the police entered, it still doesn't mean that she couldn't have been the one responsible. Or is there something else which means that if she was guilty then she had to be alive when the police entered?