Biliary Dyskinesia: Given the patient's symptoms of postprandial worsening of pain, particularly after fatty meals, and a slightly reduced gallbladder ejection fraction on HIDA scan, biliary dyskinesia is a plausible diagnosis. This condition involves impaired gallbladder contraction, which can lead to symptoms similar to those the patient is experiencing.

Irritable Bowel Syndrome (IBS): Although the patient's pain is primarily in the upper abdomen, IBS can sometimes present with upper abdominal pain, especially if the patient has a subtype that involves upper GI symptoms. The exacerbation of symptoms by certain foods could also fit with IBS.
Functional Dyspepsia: This condition involves recurring or persistent upper abdominal pain or discomfort that is not explained by other pathologic conditions. Given the patient's postprandial worsening of symptoms and the lack of findings on extensive workup, functional dyspepsia is a consideration.
Gallstones or Cholecystitis not detected by initial imaging: Despite the initial workup, it's possible that small gallstones or early cholecystitis were missed. The patient's symptoms after eating fatty meals could suggest a gallbladder pathology that was not fully captured by the initial diagnostic tests.

Peptic Ulcer Disease: Although the patient had an EGD, it's crucial to consider peptic ulcer disease, especially if the initial endoscopy might have missed an ulcer or if a new ulcer has developed. The patient's use of NSAIDs (like Toradol) could contribute to this risk.
Pancreatitis: Acute or chronic pancreatitis can present with upper abdominal pain radiating to the back and can be exacerbated by fatty meals. This diagnosis is critical to consider due to its potential severity and the need for specific management.
Hepatic or Renal Pathology: Conditions affecting the liver or kidneys, such as hepatitis, liver cysts, or renal stones, could present with upper abdominal pain and need to be considered to avoid missing a potentially serious condition.

Celiac Disease: Although less common, celiac disease can cause abdominal pain and could be considered if the patient's symptoms are related to gluten intake.
Gastric or Duodenal Diverticulum: These are rare conditions that could cause abdominal pain and might not have been detected on initial imaging or endoscopy.
Intra-abdominal Malignancy: Although very rare in a young patient, intra-abdominal malignancies can cause persistent abdominal pain and should be considered if other diagnoses are ruled out and symptoms persist.