Sorry for your loss. If you still read replies: The problem in this case was likely electrical, whereas the stress test is usually meant to uncover problems that arise from inadequate blood flow in coronary arteries. Myocarditis (heart muscle infection) can cause "heart block", which can lead to cardiac arrest. Myocarditis is notably hard to diagnose, unfortunately with the surest means being an autopsy. Otherwise there would need to be enough scoring on the Dallas Criteria to warrant literally taking a snip of heart muscle for biopsy.

"The myocarditis was focal and mild in the sections examined, but cannot be completely excluded as a contributing factor."

Which means it was not widespread, besides being mild. That also means a biopsy might not have found it, except by chance if a snip was taken from an infected spot.

Ironically, the exercise on the treadmill might overcome heart block and give a more normal picture on the EKG.

As you can see, it is complex. One possible avenue: if the myocarditis (usually painless) was accompanied by pericarditis, which is known for chest pain. Yet you didn't mention pericarditis. Then you'd have to show that they should have been able to distinguish pericarditis from the angina that comes with blocked arteries - which is what they tested for.

I hope you're seeing an atty that specializes in cardiac cases, else he won't know how to begin evaluating this and whether he should incur the expense of consulting med experts - especially with cause of death being so unsure.