Is there a hadith that permits praying upon the Prophet (peace and blessings be upon him) in congregation and audibly after the prayer, given the issue of it interrupting dhikr or the recitation of the Quran, and what is the ruling on congregational and audible seeking of forgiveness after the prayer?

There is no authentic hadith regarding the virtue of sending blessings upon the Prophet, peace and blessings be upon him, after reciting Ayatul-Kursi following the prayer. The guidance of the Prophet, peace and blessings be upon him, and his Companions was contrary to that. Rather, the place for sending blessings upon him is in the Tashahhud. Collective dhikr after the prayer is considered among innovations, and this includes collective seeking of forgiveness, because it is an adherence to matters that are not obligatory, from which legislation is understood. As for interrupting recitation or dhikr to send blessings upon the Prophet, there is no harm in it. It is permissible to delay that until after Ayatul-Kursi or to suffice with doing it once.