With diarrhea, fever, chest pain, EKG ok, echo ok, prescription ok, troponin 300. Myocarditis confirmed?

No. With massive troponin elevation (300!), you have to rule out coronary occlusion (thrombosis/dissection/embolism) - need coronary angiography to rule that out. Cardiac MRI is sometimes suggestive of myocarditis, but most cardiologists would still need a coronary angiogram to rule out coronary abnormalities.