What is meant by Sheikh Muhammad 'Illaysh's statement in "Minah Al-Jalil": "And the predominance of doubt is not stipulated for one afflicted by doubt due to his inability to achieve it, and doubt suffices for him therein," and to what extent is it valid to act upon it?

In summary, for rubbing, it is not necessary to achieve a غالب الظن (preponderance of belief) that water has reached the limb in ghusl and wudu; mere doubt is sufficient. This applies to the Mustankah (one frequently afflicted by doubt) who experiences doubt daily or once or twice a day. They are not required to follow up on it but should disregard it. As for someone who is not a Mustankah, they are obligated to have a غالب الظن (preponderance of belief) that water has reached the limb, because they do not suffer from obsessive whispering (waswasa). And Allah knows best.