Idiopathic Pulmonary Fibrosis (IPF): Given the patient's age and presentation with bilateral reticulonodular shadows, IPF is a strong consideration. The history of unilateral pleural effusion could be related to the progression of the disease or a complication thereof.

Pneumonia: Bacterial, viral, or fungal pneumonia could present with bilateral reticulonodular shadows and lead to ARDS, especially in an elderly patient. The recent history of pleural effusion could be indicative of a complicated pneumonia.
Connective Tissue Disease (CTD) associated Interstitial Lung Disease (ILD): Conditions like rheumatoid arthritis, scleroderma, or lupus can cause ILD, which might present similarly. The unilateral pleural effusion could be a manifestation of the underlying CTD.
Chronic Hypersensitivity Pneumonitis: This condition can cause bilateral reticulonodular infiltrates and has a varied presentation, including pleural effusions in some cases.

Malignancy (Lymphangitic Carcinomatosis): Although less likely, metastatic cancer to the lungs can cause bilateral reticulonodular shadows and pleural effusions. Missing this diagnosis could be fatal.
Tuberculosis (TB): Especially in regions with high TB prevalence, TB can cause a wide range of pulmonary manifestations, including reticulonodular shadows and pleural effusions. It's crucial not to miss this diagnosis due to its treatability and public health implications.
Acute Interstitial Pneumonia (AIP): A rare but severe form of ILD that can present with ARDS and has a high mortality rate if not promptly recognized and treated.

Eosinophilic Pneumonia: Characterized by eosinophilia and pulmonary infiltrates, this condition can present with a variety of symptoms, including pleural effusions.
Alveolar Proteinosis: A rare condition that can cause bilateral reticulonodular shadows due to the accumulation of surfactant-like protein and phospholipids in the alveoli.
Lymphoproliferative Disorders: Such as lymphoma, which can involve the lungs and cause a range of pulmonary manifestations, including reticulonodular shadows and pleural effusions.

The workup should include:

High-resolution CT (HRCT) scan of the chest to better characterize the lung parenchyma and pleura.
Pulmonary function tests (PFTs) to assess for restrictive or obstructive patterns.
Bronchoalveolar lavage (BAL) to analyze the cellular and biochemical composition of the lung secretions.
Serological tests for connective tissue diseases and autoimmune markers.
Infectious disease workup, including TB testing and fungal serologies, depending on the clinical context and epidemiological risk factors.
Consideration for lung biopsy if the diagnosis remains unclear after initial evaluations, to differentiate between various causes of ILD and to rule out malignancy.