Why is Imam al-Hasan ibn 'Ali not included in the list of Ismaili Imams?
The Nizari Isma‘ilis do not include the name of al-Hasan ibn ‘Ali in their list of Imams which has led some people to conclude that al-Hasan is not accepted as an Imam in Nizari theology. In reality, al-Hasan ibn ‘Ali is regarded as an Imam by the Nizaris but with a minor difference: al-Hasan is understood to be an Entrusted Imam or Trustee Imam (al-imam al-mustawda) as opposed to a Permanent Imam (al-imam al-mustaqarr), the latter position belonging to his brother al-Husayn ibn ‘Ali. The Nizari list of Imams only includes the names of the Permanent Imams and not the Entrusted Imams.
The difference between the Entrusted Imam and the Permanent Imam is that the Entrusted Imam is a brother or cousin of the main genealogical line of the Imams who holds the rank and authority of Imam for a temporary period and the Imamate does not permanently dwell among the Entrusted Imam’s descendants. The Permanent Imam is the hereditary Imam who inherits the Imamate from his forefathers and transmits it to his descendants. The Entrusted Imam is only appointed in special circumstances and is usually the brother or cousin of the Permanent Imam. This is all according to the teachings of Imam al-Mu'izz and repeated by later Ismaili Imams and Da'is.
When there is an Entrusted Imam, the Permanent Imam remains silent (samit) although he is the source of authority (amr) of the Entrusted Imam who acts on his behalf. Thus, Imam al-Hasan was an Entrusted Imam as he held the authority and rank of Imamate after Hazrat ‘Ali and then bequeathed it to his brother Imam a-Husayn who then transmitted the Imamate in his progeny. On a similar note, the Prophet Muhammad, during his mission, was the Entrusted Imam in his time while Imam ‘Ali ibn Abi Talib and his forefathers were the Permanent Imams.
In Nizari Isma’ilism, al-Hasan ibn ‘Ali also holds the rank of pir or supreme hujja which is the rank in the Isma’ili hierarchy (hudud) second only to the Imam himself. This has led some to confuse the positions of Entrusted Imam and pir or simply deny that al-Hasan was an Imam altogether. In reality, al-Hasan was both an Entrusted Imam and a pir (supreme hujja) and this is perhaps why the Nizari Isma‘ili Ginans, the Asal Du’a (Old Du’a) and the farmans of Imam Sultan Muhammad refer to al-Hasan as ‘Pir Imam Hasan’.
Shafique Virani, The Isma‘ilis in the Middle Ages, pp. 83-85