Torsades de Pointes: Given the patient's history of congenital long QT syndrome, this condition is a well-known complication that can lead to palpitations and chest pressure due to the abnormal heart rhythm. The eating disorder may exacerbate the condition through electrolyte imbalances.

Hypokalemia-induced Arrhythmia: Eating disorders can lead to severe electrolyte imbalances, including hypokalemia, which can trigger arrhythmias in someone with long QT syndrome.
Anorexia Nervosa-related Cardiomyopathy: The eating disorder could have led to malnutrition and subsequent cardiomyopathy, which might present with chest pressure and palpitations.
Dehydration and Electrolyte Imbalance: Dehydration and electrolyte disturbances are common in individuals with eating disorders and can precipitate arrhythmias, including in those with long QT syndrome.

Myocardial Infarction: Although less common in young individuals, myocardial infarction should not be missed, especially if there are risk factors such as severe malnutrition affecting heart health.
Pulmonary Embolism: This is a critical diagnosis to consider, especially in patients with significant dehydration or those who may be at risk for deep vein thrombosis due to malnutrition and immobility.
Cardiac Tamponade: Any condition leading to significant cardiac dysfunction could potentially lead to tamponade, which is life-threatening and requires immediate intervention.

Mitral Valve Prolapse: While not directly related to long QT syndrome or eating disorders, mitral valve prolapse can cause palpitations and chest discomfort. It's less likely but should be considered in a broad differential.
Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): A rare condition that affects the heart muscle and can lead to arrhythmias and sudden death. It's less common but could be considered, especially with a history of palpitations and a potential family history of sudden death.