Carotid Artery-Cavernous Sinus Fistula or Thyroid Ophthalmopathy: Given the combination of chemosis, diminution of vision, watering, diplopia, pain on eye movements, and ptosis, these conditions are high on the list due to their potential to cause a range of ocular symptoms. However, the normal pupillary reaction and the specific pattern of symptoms might lean more towards a condition affecting the orbit and extraocular muscles directly.

Orbital Pseudotumor (Idiopathic Orbital Inflammatory Disease): This condition can cause pain, swelling, and limitation of eye movements, which aligns with some of the patient's symptoms.
Graves' Ophthalmopathy: Although the patient's vision improves with pinhole (suggesting a refractive issue), Graves' can cause proptosis, diplopia, and eyelid retraction (though ptosis is less common), and it's a consideration given the constellation of symptoms.
Orbital Cellulitis: Infection within the orbit can cause severe pain, especially with eye movement, proptosis, and decreased vision, but the absence of fever or significant eyelid swelling might make this less likely.

Giant Cell Arteritis: This condition can lead to sudden vision loss, especially in older adults, and while the patient's vision is not severely impaired, the potential for rapid progression to blindness makes it a critical diagnosis not to miss.
Cavernous Sinus Thrombosis: A life-threatening condition that can cause orbital symptoms, including chemosis, proptosis, and cranial nerve palsies, which could explain the diplopia and ptosis.
Orbital Apex Syndrome: A condition where a mass or infection at the orbital apex can cause a range of symptoms including vision loss, diplopia, and ptosis due to involvement of the cranial nerves.

Tolosa-Hunt Syndrome: A rare condition characterized by painful ophthalmoplegia due to nonspecific inflammation of the cavernous sinus or superior orbital fissure, which could explain some of the patient's symptoms.
Orbital Myositis: Inflammation of the extraocular muscles, which can cause pain, diplopia, and limitation of eye movements, but is less common and might not fully explain the range of symptoms presented.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A systemic vasculitis that can involve the orbit and cause a variety of symptoms including vision loss, diplopia, and orbital pain, but is relatively rare.