Its not true to say that most Coroners have BOTH medical and law degrees. The majority are from a legal background (many have experience in medical negligence or inquests) and of course they then undertake some further training to become a Coroner.
Also it would be very unusual for the actual medical cause of death to be changed in an inquest. Before an inquest is opened a Coroner will have either accepted the cause of death reported on the medical certificate by the doctor / hospital, or if that's not possible, they will order a post mortem, and the pathologist will confirm cause of death. Of course in a very small number of cases a medical cause of death is never established.
An inquest seeks to reach a conclusion about how someone came by their death e.g Natural Causes, Accident, Misadventure. The medical cause of death is one piece of evidence which assists the Coroner in reaching that conclusion.