What is the ruling concerning the prayer of congregants behind an imam, if it is later discovered that his prayers in which he led them were invalid due to his not having completed purification, out of ignorance on his part?

The prayer of the congregants is valid, and they are not required to repeat it. The Rightly Guided Caliphs led the people in prayer, and then realized they were in a state of ritual impurity after the prayer. They repeated their own prayers but did not command the people to repeat theirs. As for you, you must repeat your prayer until you achieve certainty or strong presumption that you have fulfilled your obligation.