Im sorry this happened to you. My father died In A similar manner, playing tennis. He was in great health, it was a terrible shock to our family and left me fatherless at a young age. Your father likely had a cardiac arrhythmia, it could be related to the Covid virus as it can settle on your heart. If there is no cause of death found at the autopsy it will fall under the category of sudden cardiac death. Resuscitation is not like in the movies and has a low likelihood of good outcomes especially in the field (outside of the hospital). Sometimes despite everyones best efforts people still pass, do not blame yourself or anyone else as these are just acts of nature or God.
Cardiac incidents are complicated. You mention that he had plaque (calsium deposit) in his heart, which can cause heart attacks, he might have been revived at first and then suffered another attack later causing him to pass away. Having plaques in the arteries of the heart is one of the main causes of heart attacks, and if a piece of this build up breaks off a blood cloth might form around it, blocking the blood flow to the heart starving it for oxygen which is a potentially life threatening condition.That's a question better fit for a Cardiologist, as an EMT it's a little outside my job description to know the fine details about the tests. I know they use duplex ultrasonography which is an imaging tool that uses sound waves to check the flow of blood in the veins. It can detect blood clots and blockages in the deep veins. But I'm not sure how much/little build up there has to be for it to detect it. They also use a blood screening called "D-dimer" which is a substance that is released when a blood clot breaks up. Again, I'm not sure how sensitive the test is.