If he just had a heart attack odds are the autopsy won’t show anything (other than a blocked artery), a recent attack will show soft necrotic tissue, and a healed MI should show scar tissue.
Sounds more like a sudden cardiac arrest rather than a heart attack (myocardial infarction). Sudden cardiac arrest is more likely to drop someone like a sack of potatoes.
then the coroner should have already released a preliminary determination with ongoing toxicology tests....no preliminary cause is in and of itself suspicious