Is one who decides not to repeat fasting and prayer due to obsessive-compulsive whispers (waswas) sinful, knowing that these whispers trouble him in all acts of worship, according to the Shafi'i school of thought?

The basic principle is the validity of the fast, and doubt concerning its invalidation does not nullify it. The fast is not invalidated merely by doubting whether a fast-breaking substance has reached the stomach. Therefore, not making up the fast and disregarding such doubts is the correct decision.