Infectious Mononucleosis (Mono): Given the patient's overall not feeling well, tender neck, and the presence of + EBV antibodies, infectious mononucleosis is a strong consideration. The mildly enlarged lymph nodes and the clinical presentation are consistent with this diagnosis.

Reactive Lymphadenopathy: The MRI findings of multiple bilateral jugular chain lymph nodes that are more notable for number rather than size, along with a few mildly enlarged nodes, could be indicative of a reactive process. This is often seen in response to viral infections or other benign conditions.
Viral Pharyngitis: The patient's symptoms of a tender neck and not feeling well could also be consistent with viral pharyngitis, which can cause lymphadenopathy.

Lymphoma: Although the MRI findings are favored to be benign/reactive, malignant adenopathy, such as lymphoma, could present similarly. It is crucial to follow up clinically to document stability or changes in the lymph nodes.
Tuberculosis (TB): TB can cause lymphadenopathy and systemic symptoms. Although less common in this demographic, it is a critical diagnosis not to miss due to its potential severity and the need for specific treatment.
Hodgkin Lymphoma: This is another malignancy that could present with lymphadenopathy. Given the patient's age and symptoms, it is essential to consider and rule out this diagnosis.

Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymph nodes, which can cause lymphadenopathy and systemic symptoms.
Kikuchi-Fujimoto Disease: A rare, self-limiting condition that can cause lymphadenopathy, often accompanied by fever and night sweats.
Sarcoidosis: Although rare in children, sarcoidosis can cause lymphadenopathy and should be considered in the differential diagnosis, especially if other systemic symptoms are present.