Will the worshipper be held accountable to the Imam for the prayer of those led in prayer, or will each worshipper be held accountable for his prayer individually?

The Imam is responsible for his own prayer and for the prayer of all the congregants, because he is a guarantor of their prayers. Therefore, he must observe many matters for his prayer to be valid; because its invalidity may lead to the invalidity of the congregants' prayers. As for the congregants' failure in their prayers, the Imam is not responsible for it; rather, each congregant is responsible for himself.