What is the ruling on the prayer of one who urinated then performed ablution, forgetting to wash his private parts, and led the congregational Maghrib and Isha prayers? And what is the ruling on the prayer of those led by him?

The prayer of one who performs ablution (wudu) but does not clean himself after defecation or urination (istinja) is valid because istinja is not a condition for the validity of wudu. The prayer of one who prays with impurity, either out of ignorance or forgetfulness, is valid. The questioner's prayer is valid for him and for anyone who prays behind him.