Is it permissible, Islamically and health-wise, to use a single vessel for Istinja' (cleaning oneself after relieving oneself) and then for Wudu' (ablution), given that someone with Waswas (obsessive doubts) cleans the vessel multiple times with cleaning agents after Istinja'? And is the water that splashes from the bathroom floor onto one's leg considered ritually impure (najis)?

The cure for obsessive thoughts (waswas) is to disregard them and pay them no mind. It is permissible to use a single vessel for istinja (definition)' (washing after relieving oneself) and wudu' (ablution). A vessel is not deemed impure except by certain conviction, and its original state is purity. If a vessel becomes impure, washing it with water is sufficient to purify it. Water splashing from the bathroom floor is pure; its impurity is not ruled except by certain conviction. One must abandon obsessive thoughts and build upon the principle of purity until certain conviction to the contrary is achieved.