Part of the tops of cornea have lost color (brown) after eyes shrunk in sockets due to severe dehydration. What is happening? Retinal detachment?

See a doctor. It is not normal to have tops of the cornea without color after eyes shrunk. It is not normal for the eyes to shrink because even when the eye pressure is near zero, the eye maintains shape. Retinal detachments have entirely different symptoms. You seriously need to see an eye specialist.
Arcus senilis. I presume you are referring to a whitish grey ring in the cornea that appears all the way around the colored part of the eye. Most commonly this is not due to cholesterol. It is a normal finding in people as the age, called arcus senilis. It can be a sign of elevated cholesterol in people younger than 45. It is more visible/noticeable in individuals with dark coloring of the eye.
Interesting question. Not sure what you mean. Corneas are clear. With age, the cornea can have cholesterol build up giving the cornea a "blue" color. Less commonly, a brown ring can develop if a person has too much copper with wilson's disease. Dehydration would not explain what you are seeing. Neither would retinal detachment unless the retinal detachment is long standing. The person you describe should get an i exam.