Is a partial ghusl sufficient for ritual purity, even when one knows the complete ghusl? Does spreading water over the body mean pouring it all at once? Is ghusl valid if there is Vaseline on the hair? What is the proof that doubt is dispelled by certainty?

The sufficient ghusl (ritual bath) is enough to achieve purity, even if a person knows the complete ghusl; this is based on the Prophet's (peace and blessings be upon him) statement to Umm Salama: "It is sufficient for you to pour three handfuls of water over your head, and then pour water over the rest of your body." The intention behind wetting the entire body is for water to reach all parts of the body, and it is not a condition for it to be done all at once. Since the question indicates obsessive doubts (waswasah), it is recommended to supplicate, humble oneself, distract oneself from these whispers, consult a trustworthy psychiatrist, and have a good opinion of Allah, for Allah has not placed any hardship in religion.