What is the ruling on the imam leading people in prayer not from the mihrab or the first and second rows, due to the heat, and is it permissible for a worshipper to pray in front of the imam in the first row?

There is no harm in the Imam being late to the mihrab out of necessity, such as heat. As for what the ma'mum (follower) does by praying in front of the Imam, this is not appropriate. His prayer may be invalidated according to some scholars, and it is disliked by others for no necessity, as it contravenes the Sunnah. The first row is always the one directly behind the Imam.