What is the ruling regarding a wall painted with mastic mixed with water used for istinja' (ablution after defecating or urinating) in an unpurified vessel, and then the drips are reapplied to the wall? And how is it purified?

The water hose and the container are pure, because their default state is purity, and mere doubt does not render them impure. Likewise, the wall does not need purification, because the default state of things is purity until impurity is proven with certainty. It is advisable to beware of falling into obsessive doubts.