What is the ruling regarding the Imam rising from Rukoo’ (bowing) if he nullifies his Wudu’ during it, and what is the correct method of appointing a successor, and does the worshiper complete his prayer individually if he was leading only one person, and does he then attain the reward of congregational prayer?

If the Imam has a hadath (breaks his ritual purity) while bowing, he should rise from his bowing position without saying "Sami' Allahu liman hamidah" (Allah hears him who praises Him) so that the congregants do not rise with him. The successor Imam will then lead them after taking the Imam's place. The prayer of the congregants is valid even if they complete it individually, and they will receive the reward of congregational prayer if they catch one rak'ah with the Imam.