Is saying, "I am the rabbi of foolishness" an act of disbelief (kufr), and is reciting the Shahada after an unconfirmed statement of disbelief considered doubt in one's faith?

A Muslim should guard his tongue, and this is especially true for the one afflicted with waswas (obsessive whispering). What you uttered does not lead to disbelief, and your pronouncement of tawhid (definition) (Oneness of God) and seeking forgiveness will benefit you, not harm you. The one afflicted with waswas should consider himself a Muslim, for whoever enters Islam with certainty does not exit it except with certainty. The cure for waswas is to completely disregard it.