Tension Headache or Migraine: Given the description of right-sided headache occurring almost every night without any specific findings on the MRI, tension headache or migraine are plausible diagnoses. These conditions are common and can be exacerbated by various factors including stress, sleep patterns, and sinus disease, which is hinted at by the minor mucosal thickening of the paranasal sinuses.

Sinusitis: The minor mucosal thickening of the paranasal sinuses suggests sinus disease, which could be contributing to the patient's headaches. Sinusitis can cause referred pain to the face and head, including the sides, depending on which sinuses are affected.
Temporal Arteritis: Although less common, temporal arteritis (giant cell arteritis) could be a consideration, especially if the patient is over 50. However, the absence of specific symptoms like jaw claudication, visual disturbances, or tenderness over the temporal arteries makes this less likely.
Trigeminal Neuralgia: This condition involves sudden, severe, shock-like pain around the eyes, lips, or nose mainly because of abnormal blood vessels pressing on the trigeminal nerve in the head. The history of melanoma might raise concerns about metastasis affecting nerves, but the MRI did not show specific lesions.

Metastatic Disease: Despite the MRI not showing any intracranial lesions, given the patient's history of melanoma, it's crucial not to miss potential metastatic disease. Melanoma can metastasize to the brain, and such metastases can sometimes be subtle or not visible on non-contrast MRI. The decision to forgo contrast might limit the detection of certain lesions.
Pituitary Apoplexy or Other Pituitary Lesions: Although the pituitary is reported as normal, any sudden headache, especially if severe and acute, should prompt consideration of pituitary apoplexy, a condition where the pituitary gland suddenly hemorrhages or becomes infarcted. This is a medical emergency.
Subarachnoid Hemorrhage or Other Vascular Issues: The absence of microhemorrhage and normal appearance of structures does not entirely rule out vascular issues, especially if the patient's headache has a sudden onset or is described as "the worst headache" of their life. However, the clinical history provided does not strongly suggest this.

Idiopathic Intracranial Hypertension (IIH): This condition, also known as pseudotumor cerebri, involves high blood pressure within the spaces that surround the brain and spinal cord, which can cause headache. It's more common in obese women but can occur in anyone. The MRI findings do not support this diagnosis directly, but it remains a consideration in the differential for chronic headache.
Cranial Neurosarcoidosis: This is a rare condition where granulomatous inflammation affects the nervous system. It can cause a variety of neurological symptoms, including headache. However, specific findings on MRI, such as leptomeningeal enhancement (which would require contrast to appreciate), are usually necessary to consider this diagnosis.