What is the ruling on performing ghusl and wudu for past prayers when there is doubt about water reaching all body parts, and what is the ruling on obsessive whispers of disbelief after thinking about something?

The cure for doubts and misgivings concerning an act of worship after completing it is to ignore them and pay them no mind, for doubt does not affect the validity of the worship, and the default assumption is its soundness. As for disbelief-inducing whispers (waswas al-kufr), they do no harm and do not affect the soundness of faith, nor is there any sin in them; because Allah has overlooked for this Ummah what they whisper to themselves, as long as they do not act upon it or speak of it.