I think you may need to explain the used water part more clearly. There is water left over that the wudu was taken from, and there is also the water that his been compiled from what fell off of the persons body while making wudu. Also, the Shafii madhhab clearly states that water used after the first mandatory wash of Wudu (ablution) is not to be re used for purifying one self again. If the water was used for the second or third wash, which are Sunnah, then that water may be reused for another person’s Wudu. Also, the water that is left over in the container that the water was taken from, has two situations. 1. If the water is considered alot (there is a difference in opinion in what alot is), and it did not change to an extent that you can not call it pure water anymore, then it is ok to use for wudu and ghusl. If the water is considered to be less than that “alot” amount, and you actually put your hand or other body part while making the first mandatory wash, then the water is considered used in the shafii madhahab. Whether or not it changed. I am just making clear the shafii madhhab, because of what you stated. This is the strong opinion within the madhhab. Yes, the Malikii madhahb states that it is ok to use the leftover water that falls from a person after there wudu, but it is disliked if the water is little. If it is alot, then it is not disliked. Also, if the water that falls off of the person after he makes wudu or ghusl has changed to the point where it cannot be called water anymore, then it cannot be used again for the mandatory wudu or ghusl even if it was alot. This is also the ruling for the water that remains in the container that was used to purify oneself, if ofcoarse he put his body directly inside of the bucket instead of pouring it on to his body parts during the first mandatory wash.
Also, I have heard that the original version of fiqh as Sunnah as some inaccurate information in some places.

Wallahu A3lam