Is the congregational prayer to be established when the Imam enters the mihrab, or is it permissible to establish it while he is outside it, and what is the ruling on that?

The time for the iqama is when one intends to enter prayer, provided that the interval is not prolonged except for something recommended. The muezzin should not call the iqama until the Imam gives him permission. However, if his permission is known, then there is no harm. The muezzin of the Messenger of Allah, peace and blessings of Allah be upon him, would wait and not call the iqama until he saw the Messenger of Allah, peace and blessings of Allah be upon him, had come out, whereupon he would call the iqama as soon as he saw him. There is no difference whether the Imam is inside the mihrab or outside it.