1- One must believe in the Wilayah of all 12 Imams to have the correct Aqeedah and complete faith. If one knowingly rejects the Wilayah of the other Imams, one has failed his test and will not be saved. But this Wilayah starts with Imam Ali (a).
2- The hadith mentions Imam Ali (a) because he is the gateway to Wilayah. It starts with him. That’s like saying: if you believe in the Qur’an you will be saved. So someone only follows the Qur’an and rejects hadith. Would they be saved? The Qur’an is the gateway, but it doesn’t include all details. Does the Qur’an say we have to pray 5 times a day or that we pray 17 units? Similar idea here.
The main idea of this hadith is that Islam is a comprehensive religion. To be saved and to go to heaven, you must accept all of Islam. After the Prophet you follow Imam Ali and his teachings. And Imam Ali said after me follow Hassan and Hassan. So if you go all the way you will be saved