You have no idea what underlying conditions the kid might have had. By 17 days there is probably only trace amounts of the vaccine left in the body.
Second, you (or the original poster) can't criticize the methodology of counting Covid deaths (every person who tested positive is counted as a death) and then use the exact same methodology to count vaccine deaths.
If the vaccine causes an inflammation process there's no reason myocarditis or something similar could not last for weeks at least. I don't think a heart attack at 17 days is out of the question. You don't need actual antigen for inflammation to stay around, that's immediately obvious if you look at some people's arms and lymph nodes a week or more after vaccination.
Second, you (or the original poster) can't criticize the methodology of counting Covid deaths (every person who tested positive is counted as a death) and then use the exact same methodology to count vaccine deaths.