Is the water falling from the upper part of the body during ghusl, which separates from some limbs then returns to them, considered used water, necessitating pouring water again, or does it remain pure until it completely separates from the body, and a single pouring is sufficient to cover all limbs?

Obsessive doubts have reached a significant level in your case, and we advise you to ignore and disregard them. The prevalent opinion is that used water is pure, and it is permissible for someone afflicted with obsessive doubts to adopt the easiest of opinions.

What you have found problematic is purely obsessive doubt. Jurists have stated that the body of one performing ghusl (ritual bath) is like a single limb, and water does not become musta'mal (used) until it separates from the entire body. It is stated in Al-Bahr Ar-Ra'iq: "Indeed, they have agreed that the body in ghusl is like a single limb, and they have agreed that water does not become musta'mal except after separating from the limb. So, according to the narration of indivisibility, it does not become musta'mal unless it separates from the entire body." Imam An-Nawawi said: "The ruling of being musta'mal is only established after separation from the limb, and the body of someone in a state of janabah (major ritual impurity) is like a single limb." So, abandon these obsessive doubts.