The ruling on the prayer of one who steps forward to lead the imam during the prayer without a deputy being appointed

Q: If the imam prays part of the prayer and then interrupts the prayer, and another person takes his place and completes the prayer without a deputy being appointed, and after the prayer ends, confusion arises among the followers regarding whether a deputy was appointed or not, and some of them ask the other imam, "Did the imam appoint you as a deputy?" and he says, "No," is it obligatory for the followers to repeat the prayer? Is it obligatory to investigate if they were unaware of the situation? And are there different opinions in this issue? Because some students of knowledge have issued a ruling that the prayer is valid, while others have ruled that it is invalid?

A: If the imam appoints one of the ma'mum to lead when he needs to leave the prayer, it is permissible according to the most apparent of the scholars' opinions. This was reported from 'Umar and 'Ali (may Allah be pleased with them both), and 'Umar (may Allah have mercy on him) did it when he was stabbed while in prayer, as he appointed 'Abd al-Rahman ibn 'Awf (may Allah have mercy on him) to complete the prayer. The story is in Sahih al-Bukhari.
Likewise, if the ma'mum appoint one of them when they are few, or if some of them appoint one when they are many, or if one of them steps forward and completes the prayer without anyone appointing him. Abu Muhammad ibn Qudamah (may Allah have mercy on him) said in al-Mughni, after providing evidence for the stronger opinion, which is the permissibility of appointing a substitute: "If this is established, then the imam may appoint a substitute to complete the prayer with them, as 'Umar (may Allah have mercy on him) did . And if he does not appoint a substitute, but the ma'mum appoint one of them to complete the prayer with them, it is permissible. And if they pray individually, it is permissible. Al-Zuhri said regarding an imam who is bleeding, has a nosebleed, or finds pre-seminal fluid: 'He should leave and say, "Complete your prayer."'"
There is no doubt that one of the followers stepping forward to complete the prayer with them is more preferable than them completing the prayer individually. Appointing a deputy, whether by the imam or by the followers, is not a condition for the validity of the prayer after the imam has left it. If one of the congregation steps forward at the iqamah and prays with them without anyone appointing him, his prayer and their prayer behind him are valid. So likewise, if he steps forward during the prayer to complete the prayer after the imam has left, even if no one appointed him; because his stepping forward includes the intention of leading, and their following him includes their intention of being led by him. And because congregational prayer is required by the Shari'ah, so what achieves it is more preferable than its absence. And success is from Allah.