Hi there, Im so sorry for you and your family regarding what happened. You all must have suffered terribly with this sudden death. Its strange what it does to your minds. My father died suddenly many years ago. It seems like yesterday./ Reading your post its like my own story. You tend to beat yourself up with questions. Your father in law is probably riddled with guilt in some way. Could he have done something to save her?? Why? What could I have done? How was this not diagnosed? Its totally consuming for you all i understand.
The term on the death cert may be what the autopsy doctor felt was the cause. These would be the clinical facts. The reason this was the official reason was due to what was found on examination. The decreased function ( Its Called EF) is a symptom of Heart Failure. The 60% you mention is probably the EF number. But if the EF number was 60% , this is normal. ? EF is usually normal in a range of 55 - 70%. Its the amount of blood that leaves the heart each time it pumps. Nothing I say or anything you do will change whats happened. It must be so hard to accept. This is a brief explanation, but hope it helps. All the family need comfort and love right now. Try and remember the good times. Best wishes.