Allah (The Glorious) knows whom to appoint as an Imam and we have no authority nor capacity to know. We definitely believe that every order by Allah (SWT) is based on the absolute benefit of human beings and the whole universe.
Al-Hasan Al-Muthanna was never an Imam, as the Imamate after Imam Husain (AS) was to Imam Ali Zainul Abideen (AS). No one from Imam Hasan's sons or grand sons was an Imam.
All the names of the Twelve Imams were clearly mentioned by the Prophet Muhammad (SAWA) in many authentic Hadeeths. (Ithbaat Al-Hudaat By Al-Hurr Al-Aamily in three volumes).