Is the first washing (the obligatory one) during Wudu' sufficient if there is a slight impurity on the forehead (al-nasiyah) that is difficult to avoid? And in this case, does the water used for Wudu' become impure as it separates [from the face]? And is continuously washing the face before every Wudu' in this situation considered excessive (tanattu') or a legitimate precaution?

The default state of things is purity, and this certainty is only removed by a certainty like it. Therefore, bathroom doorknobs and other things are pure unless one is certain of their impurity. Even if their impurity is highly probable, they remain pure. If one is certain of their impurity and time has passed, then the sun and wind purify them if no trace of the impurity remains.

As for performing ablution (wudu') with impurity on a limb, some scholars hold that removing the impurity is a condition for its validity. However, the more correct view is that this is not a condition. So, if one performs ablution while impurity is on one of their limbs, and intends by washing it to remove the ritual impurity, it suffices.

The intention is not a condition for removing impurity according to the majority of scholars. So, if the impurity itself is removed or water passes over it, it is gone. And water that separates after removing impurity, if it has not changed due to the impurity, is pure.

As for washing the face before ablution out of fear of impurity, this is not from the Sunnah and is a form of waswasah (obsessive whispering), unless one is certain of the presence of impurity, in which case it should be removed to avoid scholarly disagreement.