I'm not a cadiologist, but how do you diagnose an arythmia after death? Arterial blockage? Damaged valve? Weak muscle? Yes to all three, but don't you have to watch an active EKG to diagnose the existence of an arythmia?
The coronor in this case is a shady character.
You don't. It's a supposition that should be based upon either previous knowledge of an arrhythmia in that patient, or anatomic/histological, or previous electrocardiographic criteria that predispose to arrhythmias (as you point out).
If she had, on autopsy, a cardiomyopathy that predisposed her to arrhythmia (e.g. hypertrophic cardiomyopathy, arrhythmogenic right ventricular dysplasia, a dilated cardiomyopathy, etc.) then this would be a reasonable assumption. Coronary disease would be very unusual and rare in a healthy young woman of her age, unless she had some type of congenital coronary anomaly. In any case, these would be found during autopsy.
There are conditions predisposing to arrhythmia, such as long QT syndrome, Brugada syndrome, active myocarditis, etc., but it's a reach to make these diagnoses without evidence (e.g. previous ECGs, autopsy histological data showing myocarditis, etc.).
The bottom line is that if there was adequate reason to make the diagnosis of 'arrhythmia' as the cause of death, then they should release that data and put this all to rest - for the families sake mainly.