Did Imam Hassan stipulate to Mu'awiya that the caliphate be by consultation (shura) after him, and why did Mu'awiya hand it over to his son Yazid despite the presence of venerable Companions from the Prophet's household (Ahl al-Bayt) and others?

A Muslim in this era should focus on reforming himself, his society, and his time, and cease searching for faults of the Companions, for Allah was pleased with them and purified them. It is not permissible to mention them except with the best of mentions, and to refrain from discussing their disputes, interpreting their battles as stemming from their diligent striving, as they did not intend disobedience. Indeed, a mujtahid (one who strives to form an independent judgment) is not sinful if he errs. What is narrated about their shortcomings is either false or exaggerated, and for what is true among it, they are excused, for they are not infallible. However, they possess such precedence and virtues that necessitate forgiveness for anything that might have emanated from them, for they are the best of generations.