It would appear that most of the deaths that you studied were the result of cardiovascular abnormalities. How is it possible to train for a triathlon without such problems arising well in advance of the competition?
We don't know the death rate while training for a triathlon. I know deaths have occurred but I agree it's probably less than the triathlon itself. I suspect the adrenaline rush with competition and the above factors contribute.
Why are the swimming events so dangerous from the perspective of cardiac arrest?
I think because of the delay in identification and rescue and the time to get to dry land to use resuscitation equipment. Time matters with resuscitation.
What can be done to improve the safety of triathlon competitors?
First, knowledge of risk of the race for the competitors will be helpful and hopefully allow people to take training seriously, especially for the swim. Second, if there are any concerns regarding suitability of the competitor, such as symptoms with training of chest pain, shortness of breath, syncope (passing out), family history of cardiac disease or—in athletes of older age—other cardiac risk factors. Each of the people in these groups should be fully evaluated before participation.
Whether races can mandate pre-op exams (or more testing) similar to what is done for high school and college sports and whether that would make a difference is unknown. Third, I know many race directors are interested in making races safer through the number of people on the water, staggered starts, et cetera. These efforts should help. Another question is whether some minimal proficiency for open-water swimming should in some way be evaluated prior to enrollment in the race.