What is the ruling on an individual praying behind the row if the worshipper does not respond to making space for a third congregant?

If a person finds an Imam and one congregant (ma'mum) standing to the Imam's left, then if there is space in front of the Imam, they should move forward to stand behind him. If not, they should move backward. If they do not move backward, the prayer of everyone is still valid. Al-Nawawi mentioned that the companions [of our school] said that if a second congregant comes and performs the opening takbir to the Imam's left, then if there is space in front of the Imam and no space behind the congregants, the Imam should move forward. If there is space behind them but not in front, they should move backward. If there is space both in front of them and behind them, there are two views regarding which is better: The sounder view is that they should move backward because the Imam is to be followed. The other view is that the Imam should move forward because it is the action of a single person. All of this applies if the second congregant joins while standing. However, if he joins during the tashahhud or prostration, there should be no moving forward or backward until they stand up. Moving forward or backward only occurs after the second congregant performs his opening takbir.