Viral upper respiratory infection (URI): The symptoms of cough, congestion, runny nose, and mild sore throat following a severe sore throat, with no fever and negative tests for Covid, flu, and strep, are consistent with a viral URI. The initial severe sore throat could have been the onset of the infection.

Allergic rhinitis: The symptoms of cough, congestion, and runny nose could also be indicative of allergic rhinitis, especially if the patient has a history of allergies. The lack of fever and the pattern of symptoms support this diagnosis.
Adenovirus infection: Adenoviruses can cause a range of respiratory symptoms, including sore throat, cough, and runny nose, and can present without fever. The negative tests for common pathogens make adenovirus a plausible cause.
Coronavirus (non-COVID-19): Other coronaviruses can cause similar symptoms to COVID-19 but are less severe. The negative COVID-19 test does not rule out infection with another coronavirus.

Epiglottitis: Although rare and more commonly associated with severe symptoms, epiglottitis can present with sore throat and respiratory symptoms. It is a medical emergency due to the risk of airway obstruction.
Lemierre's syndrome: This condition, characterized by a bacterial infection of the throat and subsequent sepsis, can present with sore throat and respiratory symptoms. It is rare but potentially life-threatening.
Mononucleosis: Caused by Epstein-Barr virus, mononucleosis can present with sore throat, fever (although the patient is afebrile, this can vary), and later, respiratory symptoms. It's essential to consider due to its potential for complications.

Tuberculosis (TB): Although unlikely, TB can cause chronic respiratory symptoms, including cough and sore throat. The lack of systemic symptoms and negative tests for common pathogens make this less likely, but it should be considered in the appropriate epidemiological context.
Cystic fibrosis: This genetic disorder can cause chronic respiratory symptoms, including cough and congestion. However, it would typically present with more systemic symptoms and a history of recurrent infections.
Immunodeficiency: Certain immunodeficiencies can predispose individuals to recurrent or severe infections. The presentation here is not strongly suggestive, but if the patient has a history of recurrent infections, this could be considered.