Chronic Sinusitis: The presence of small mucous retention cysts with mucosal thickening along the floor of the right maxillary sinus and mild mucosal thickening of the anterior ethmoid air cells bilaterally suggests chronic sinusitis, which can cause referred pain to the face and headaches, particularly on the right side in this case.

Trigeminal Neuralgia: Although the MRI shows normal-appearing evaluable portions of the trigeminal nerves, this condition can still be considered due to the nature of the patient's facial pain. However, the absence of a visible mass lesion or abnormality in the trigeminal nerve on MRI makes this less likely.
Vascular Headaches: Given the patient's age and the presence of nonspecific supratentorial white matter T2 signal hyperintense lesions (suggestive of vascular risk factors), vascular headaches (such as migraines or tension headaches exacerbated by vascular disease) could be a consideration.
Temporomandibular Joint (TMJ) Disorder: Although not directly shown on the MRI, TMJ disorders can cause facial pain and headaches, especially if the patient has a history of dental issues or jaw problems.

Giant Cell Arteritis: This condition, although less common, is critical to consider in older adults presenting with new-onset headaches, especially if there are symptoms like jaw claudication or visual disturbances. The absence of specific findings on MRI does not rule out this diagnosis.
Cavernous Sinus Thrombosis: Although the MRI does not show a mass lesion or acute infarct, cavernous sinus thrombosis can present with facial pain and headaches. This condition is rare but potentially life-threatening and requires prompt diagnosis and treatment.
Meningioma or Other Mass Lesions Not Visible on MRI: Although the MRI did not show a mass lesion, very small or certain types of tumors might not be visible. A follow-up or different imaging technique might be necessary if symptoms persist or worsen.

Sphenoid Sinus Syndrome: This is a rare condition that can cause facial pain and headaches due to inflammation or infection of the sphenoid sinus. It's less likely given the findings but could be considered if other diagnoses are ruled out.
Raeder’s Syndrome: A rare condition characterized by a combination of facial pain, miosis, and ptosis, typically due to a lesion in the middle cranial fossa affecting the trigeminal nerve and the sympathetic nerves. The normal appearance of the trigeminal nerves on MRI makes this less likely, but it remains a rare possibility.