Is it permissible for someone praying a supererogatory (nafl) prayer to lead another person in a fard (obligatory) prayer, and if so, should he pray the fard prayer afterwards? And if it is not permissible, how would the person being led (ma'mum) know that the imam is praying a supererogatory prayer?

The preponderant opinion is that it is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafl (supererogatory) prayer. It is not necessary to inform the follower of this, because the follower's prayer is sufficient for the fard, while the supererogatory prayer is not sufficient for the fard, and the person performing the nafl must perform the fard prayer after the nafl.