Should a worshiper interrupt a supererogatory prayer to join a congregational prayer that has begun, or should he complete his prayer and await another congregation, and would he be sinful for not interrupting his prayer?

There is no harm in shortening a supererogatory prayer and joining the congregants, or completing the prayer and waiting for the group. It is better to pray with the first congregation due to the virtue of praying at the beginning of the prescribed time. The dislike of repeating a congregational prayer applies only to a mosque with a resident Imam. As for جامع (Jami') mosques, frequented mosques, and prayer rooms in workplaces and schools, there is no dislike in repeating congregational prayers in them.