Welcome to NeuroTalk.
It sounds like your brother needs the contrast study so they can rule out neuritis which is an inflammation of the nerve. The right hand and right leg weakness sounds like he may have suffered a neck injury or trauma.
I doubt they can tell if the pharyngeal mucosal space cyst is the direct cause of the face numbness. There are other areas where this cranial nerve can be inflamed such as the neck. The recovered hand and leg weakness suggests the neck was involved.
Did he suffer any trauma like a car accident or fall?
An imaging study of his cervical vertebra may be in order.
Sounds like all we have are more question than answers. The steroids usually show at least some improvement quickly. If he starts to slowly improve, that is usually consider a confirmation of the diagnosis. Nerve healing can be slow. He needs to have good B-12 and folic acid levels for nerve healing. A 500 to 1000 mcg daily of B-12 plus a B-50 complex daily would be good.
As Jo*mar said, has he had seizures?
Let us know how else we can help.
My best to you and your brother.
Mark in Idaho
58 years old, retired due to disability, married 33 years, father of three, grandfather of four, Suffered a serious concussion at 10 years old (1965) stopped most driving after last concussion at 46 years old (2001), Post Concussion Syndrome/Multiple Concussion/Impact Syndrome with PTSD, immediate and short term visual and auditory memory problems, slowed processing speed, visual and auditory processing difficulties, insomnia, absence seizures, OCD, 14 concussions since first concussion at 8 years old, Taking paroxetine for 14 years and gabapentin for 12 years. Added L-Tryptophan and successfully stopped all paroxetine after 3 months of tapering down
"Be Still and Know That I am God" Psalm 46:10