Pulmonary Fibrosis or Interstitial Lung Disease: The key findings that point towards this diagnosis include slowly progressing shortness of breath, decreased lung compliance (50% of predicted value), and hypoxemia (PaO2 52 mmHg, SpO2 83% on room air) without significant airway resistance. These features are characteristic of restrictive lung diseases, where the lungs' ability to expand is impaired, often due to inflammation or fibrosis.

Chronic Bronchitis: Although measures of airway resistance are within normal limits, chronic bronchitis could still be considered, especially if the patient has a history of smoking or exposure to pollutants. However, the lack of increased airway resistance and the prominent decrease in lung compliance make this less likely.
Pulmonary Edema: This condition, often associated with heart failure, could explain the shortness of breath and hypoxemia. However, the gradual progression over a year and the specific finding of reduced lung compliance without mention of fluid overload or cardiac dysfunction make it less likely.

Pulmonary Embolism: Although less likely given the gradual progression of symptoms, pulmonary embolism can present with hypoxemia and shortness of breath. It's a critical diagnosis to consider due to its high mortality rate if untreated.
Heart Failure: Leading to pulmonary edema, heart failure is a condition that could explain the symptoms. It's essential to assess cardiac function to rule out this possibility.

Neuromuscular Disorders: Conditions like amyotrophic lateral sclerosis (ALS) or muscular dystrophy can lead to respiratory failure due to weakness of the respiratory muscles, affecting lung compliance and leading to hypoxemia.
Lymphangitic Carcinomatosis: A rare condition where cancer spreads to the lymphatic vessels in the lungs, leading to shortness of breath and hypoxemia. This would be an unusual presentation but should be considered in the differential diagnosis, especially if there's a known history of cancer.