Though I might agree with your coverage of the "waterfront" of possibilities, it would require evidence to know which of those was true. Probability of 33.3%, 33.3%, 33.3% doesn't tell us much.
It seems possible that the coroner was responding to a family wish not to see Loretta's name be dragged through anything that looked or smelled like mud. And the gentle-but-corruptible coroner perhaps "saw it their way."
Or maybe rather than family, it was more like a political "famiglia".
HF
The way this usually works, after an autopsy that does not find a conclusive cause of death, something has to be conjectured and put on paper. It's just the way it is. So, if there is no atherosclerosis or a clot in a coronary, no evidence of acute stroke or intracranial blood, no evidence of a pulmonary embolus, etc. etc., then a diagnosis of sudden death is often made - and an arrhythmia inferred. There's nothing nefarious about this as such, and there's not a whole lot else you can say without definitive autopsy findings.
That said, the coroner would have to be under a lot of pressure (IMHO) to alter a death certificate for any reason other than a change of professional opinion about the cause of death.