Must one who is overcome by the whispers of disbelief perform ghusl and shahada, or are shahada and istighfar sufficient, and is the shahada valid if recited without intention?

If it is merely a whisper (waswasah), then pay no attention to it and turn away from it, and do not let Satan get a hold of you. This is because you do not exit the fold of Islam merely by these whispers; rather, you must struggle against them. You are not required to utter the two Shahadas, nor to perform ghusl (definition) because of them. Our advice is to turn away from these whispers, for indulging in them leads to great evil. Instead, frequently repeat the word of Tawhid (definition) (Oneness of God) as an act of worship to Allah, not with the intention of re-entering Islam.