Reactive lymph node: Given the patient's symptoms of a sore throat localized to the right anterior neck, elevated CRP, and the absence of dysphagia, a reactive lymph node due to a viral or bacterial infection (despite the negative throat culture for GAS) is a plausible explanation. The indeterminate EBV VCA IgM and reactive EBV VCA IgG and anti-EBNA-1 IgG suggest past infection, which could be contributing to the reactive lymph node.

Viral pharyngitis: The patient's presentation of a sore throat without dysphagia and the laboratory findings (elevated CRP, negative monospot, and EBV serology indicating past infection) could also suggest a viral etiology for the pharyngitis.
Tooth or gum infection: Although not directly mentioned, infections of the teeth or gums could refer pain to the neck and cause localized tenderness, making this a consideration given the right-sided sore throat.

Lymphoma: Although less likely, lymphoma could present with lymphadenopathy (enlarged lymph nodes) and systemic symptoms like malaise. The normal neck ultrasound does not entirely rule out lymphoma, especially if the involved nodes are not significantly enlarged or are in a location not well visualized by ultrasound.
Thyroiditis: Subacute thyroiditis could cause neck pain and malaise. The absence of thyroid function tests in the provided information makes this a diagnosis that should not be missed, as it can present with neck pain and elevated inflammatory markers.
Tuberculosis (TB): TB can cause lymphadenitis (infection of the lymph nodes) and present with systemic symptoms like malaise. Although less common, it's a critical diagnosis not to miss due to its public health implications and the need for specific treatment.

Kikuchi-Fujimoto disease: A rare, self-limiting condition that causes lymphadenitis, typically presenting with fever and neck pain. It's more common in young women and could be considered in the differential given the patient's demographics and symptoms.
Sarcoidosis: A systemic granulomatous disease that can cause lymphadenopathy and systemic symptoms. It's less likely but should be considered in patients with unexplained lymphadenopathy and systemic symptoms, especially if other diagnoses are ruled out.