Myocarditis associated with these vaccines according to multiple sources (peer reviewed) are self limited, and not fatal. the incidence if myocarditis is 5.6 / 100,000 all comers under 18.
There would not be many studies of a new intervention.
Not sure what you mean, however the N is quite large, and the number of cases is quite small. This was recorded in the JAMA edition a month ago. The numbers are solid.
This has nothing to do with the discussion around the vaccines, I am just presenting the data as it is known.