Is merely thinking of oneself as one of the "Companions of the Left Hand" or doubting the actions of prophets like Yusuf and Adam (peace be upon them) considered disbelief (kufr), and can self-talk and disbelieving fantasies lead one to fall into disbelief?

A person does not commit disbelief through the imaginings and thoughts of the self that result from waswasa (whisperings/obsessive thoughts). It is advisable to stop giving in to Satan and to occupy oneself with learning, worship, remembrance [of God], and supplication. One must adhere to proper etiquette when speaking about the Prophets, and believe that Allah forgave Adam, and that the Prophets are infallible regarding major sins and in conveying [God's message], though minor sins may occur from them, which they do not persist in. The statement about Yusuf is incorrect, and the verse mentioned in his story indicates the permissibility of achieving objectives through stratagems if they do not contravene Islamic law or undermine a fundamental principle.