What is the ruling on direct conversation after prayer and before completing the istighfar (seeking forgiveness), while there are worshippers making up missed parts of their prayer, which might disturb them?

It is preferable for the Imam or teacher to wait until those who missed the beginning of the prayer have completed their prayers before starting the lesson, and then to speak with a moderately raised voice. Scholars have differed on the issue of raising one's voice for teaching in the mosque; some, like Imam Malik, disliked it, while others, like Imam Abu Hanifa, permitted it. Some Maliki scholars made an exception for raising one's voice for teaching when there is a need for conveying knowledge. The preponderant opinion is the permissibility of raising one's voice for teaching when there is a need to convey knowledge, provided that it is limited to the extent of the need and that one waits until those who missed the beginning of the prayer have completed their prayer.