Is the Maghrib prayer valid if the Imam recited the Quran aloud in the third rak'ah instead of the second, performed the prostration of forgetfulness (sujud al-sahw) before the tashahhud, and then the congregants also performed the prostration of forgetfulness after the prayer?

Reciting aloud or silently in prayer is a Sunnah. One who omits it is not required to perform the prostration of forgetfulness (sujud as-sahw). However, it is recommended for him to prostrate, due to the general nature of the hadith: "If one of you forgets, let him prostrate twice." The prayer of the Imam and the congregants is valid, and they are not required to repeat it if they added two prostrations to the prayer out of ignorance regarding the place for the prostration of forgetfulness. Adding to the prayer out of ignorance does not invalidate it, based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended." The congregants' prostration with the Imam for the prostration of forgetfulness is obligatory upon them, due to the Prophet's (peace be upon him) saying: "The Imam is only appointed to be followed." The questioner is advised to seek religious knowledge and the jurisprudence of acts of worship.