Transient Ischemic Attack (TIA): Given the transient nature of the symptoms (lasting 30 minutes and resolving), the involvement of the right upper extremity, and occasional visual field disturbances, a TIA is a strong consideration. The fact that the MRI non-contrast shows no stroke and the carotid duplex shows less than 50% narrowing of the left ICA does not rule out a TIA, as the cause could be embolic from a source other than significant carotid stenosis or could be related to small vessel disease not visible on the provided imaging.

Migraine with Aura: Although the patient has no history of migraines, new onset migraines can occur at any age. The visual field disturbances could represent an aura, and the numbness could be part of a migraine equivalent. The absence of headache does not rule out this diagnosis, as migraines can be acephalic.
Peripheral Neuropathy: This could be a consideration if the numbness is confined to specific nerve distributions, but the pattern described (from hand to arm) and the association with visual disturbances make this less likely.
Cervical Spondylosis or Radiculopathy: Could cause arm numbness and weakness, but the transient nature and the visual field disturbances are not typical for this condition.

Giant Cell Arteritis: This condition can cause visual disturbances and, less commonly, neurological symptoms like numbness. It's crucial to consider this diagnosis due to its potential for irreversible vision loss if not promptly treated.
Carotid Dissection: Although the carotid duplex shows less than 50% narrowing, dissection can occur without significant stenosis and can cause transient neurological symptoms.
Cardiac Source Embolism: Conditions like atrial fibrillation can lead to embolic events causing TIAs. It's essential to rule out cardiac sources of embolism.

Multiple Sclerosis: Could present with transient neurological symptoms, but the pattern described and the age of onset make this less likely.
Vasculitis (e.g., Primary Central Nervous System Vasculitis): A rare condition that could cause a variety of neurological symptoms, including transient events. It's a diagnosis of exclusion and would require further investigation.
Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS): A rare mitochondrial disorder that can cause stroke-like episodes, but it typically presents at a younger age and with more systemic involvement.