Is it possible to have a genetic cardiac defect leading to ventricular arrhythmia/sudden death ... even with no family history of any heart defect?

Yes: It is possible for all of your family to be heterozygous for an abnormality and therefore asymptomatic and you are homozygous and at risk. It is unlikely that you would have a problem of this nature and not have had serious symptoms or diagnostic evidence by the time you were 20. You need to get control of your anxiety. You have a normal echo and a loop recorder in place. Take your Xanax (alprazolam) and chill