Acute Viral Pharyngitis/Laryngitis: This is the most likely diagnosis given the symptoms of cough, sore throat, runny nose, and loss of voice to a hoarse sound, combined with the negative COVID and strep tests. The presence of diffuse pulmonary congestion, clear rhinorrhea, and an injected posterior oral pharynx without tonsil hypertrophy or cervical adenopathy further supports this diagnosis.

Acute Bronchitis: This could be considered given the cough and diffuse pulmonary congestion. However, the absence of other symptoms like fever or productive cough makes it less likely.
Influenza: Although the COVID test is negative, influenza can present similarly and should be considered, especially during flu season. However, the lack of systemic symptoms such as fever and myalgias makes this less likely.
Allergic Rhinitis: The clear rhinorrhea and absence of systemic symptoms could suggest allergic rhinitis, but the presence of a sore throat and loss of voice points more towards an infectious cause.

Pertussis (Whooping Cough): Although less common in adults, pertussis can present with a persistent cough and should be considered, especially if the cough is severe or if there's exposure history.
Epiglottitis: This is a medical emergency that can present with sore throat, difficulty swallowing, and voice changes. Although the patient does not have all the classic symptoms (like severe dysphagia or drooling), it's crucial to consider due to its potential severity.
Lemierre's Syndrome: A rare but serious condition that can follow a throat infection, characterized by jugular vein thrombosis and septicemia. The absence of specific symptoms like neck pain or swelling makes this less likely, but it should not be entirely ruled out without further evaluation.

Vocal Cord Lesions: Such as polyps or cysts, could cause voice changes but would not typically cause the other symptoms described.
Tuberculosis (TB): Could present with chronic cough and voice changes if there's laryngeal involvement, but this would be unusual without other systemic symptoms or risk factors.
Sarcoidosis: A systemic disease that can affect the lungs and cause symptoms like cough, but it would typically be associated with other findings like skin lesions, eye symptoms, or systemic symptoms.

Given the clinical presentation and negative COVID and strep tests, further testing might include influenza testing (if seasonally appropriate), a complete blood count (CBC) to look for signs of infection or inflammation, and possibly a chest X-ray to further evaluate the pulmonary congestion.
Consider throat cultures or PCR for other pathogens if symptoms persist or worsen.
Pertussis testing (PCR) if the cough is persistent and severe.

Symptomatic relief with over-the-counter medications for cough and sore throat.
Consider a short course of corticosteroids for laryngitis to reduce inflammation.
Antibiotics are not indicated for viral pharyngitis/laryngitis unless there's evidence of a secondary bacterial infection.
Patient education on hydration, rest, and when to seek further medical attention is crucial.