How reliable is an echocardiogram for diagnosis?

Depends. It depends on what you are trying to diagnosis. It is obviously of no use in diagnosing appendicitis. For leakage or a block in a heart valve it is very reliable. Same as for measuring how hard the muscle is contracting or whether the heart chambers are enlarged. It also depends on technical issues. Less reliable if you are 400 pounds or have lung disease.
Varies Widely. Heart muscle, valve, blood flow issues detectable by symptoms, stethoscope &/or ekg are limited; eg. Most heart problems & valve problems missed. Using us, 1-10 mhz, motion of the 4 muscle chambers, 4 valves. Aorta & blood flow become visible, measurements can be made & more issues evaluated. Quality very dependent on machine/tech/person/reader. Best always get video copies, read yourself & save.