How can a person performing ghusl avoid using used water that falls from the body parts during ghusl, especially when using a water vessel for scooping, bearing in mind that the body of the one in a state of janabah is considered as one organ, and water is only deemed used after it separates from the organ?

Water only becomes musta'mal (used) after it separates from the body. Therefore, water that falls on the chest and abdomen during the washing of the head is not considered musta'mal because it has not yet separated from the body. A small amount of musta'mal water is excused if it mixes with pure water (ma' tahur) due to the difficulty of guarding against it. Hence, one should perform ghusl (definition) (ritual bath) normally and dismiss these delusions.