There is nothing wrong in offering some Nafl (supererogatory) prayers in congregation, but this should not be a regular habit such that every regular Sunnah prayer is offered in congregation, because that is not prescribed.

If an Imam breaks his Wudu during prayer, he must stop praying and appoint someone to continue leading the congregation. If the Imam does not appoint a successor, the congregation may choose a new Imam or complete the prayer individually.