Is it permissible for the one led in prayer to interrupt the Fatihah, or the middle Tashahhud, or the dhikr after rising from rukoo’ (bowing), if the imam bows, or moves to the next rak’ah, or says “Allahu Akbar” for prostration, and does this ruling apply to all prayers?

A Muslim must worship Allah with insight and follow trustworthy scholars. The opinion mentioned by the questioner, that one should act upon, is the view of those who do not consider the recitation of Al-Fatihah obligatory for the congregant, neither in silent nor audible prayers, and this is the view of the majority. As for those who deem the recitation of Al-Fatihah obligatory for the congregant, they have a detailed explanation regarding the circumstances under which Al-Fatihah is waived for him. If the Imam stands for the third rak'ah and the congregant has not completed the tashahhud, then he must complete it and then follow the Imam, because the tashahhud is obligatory according to the more preponderant opinion. The saying: "Sami'a Allahu liman hamidah" (Allah hears him who praises Him) is obligatory according to the Hanbalis, and its place is during rising from rukoo'. If the congregant left the remainder of the tashahhud or the tahmid after the Imam's tasmi' due to ignorance of the ruling, then there is nothing upon him and his prayer is valid, because obligations are waived due to forgetfulness and ignorance according to the Hanbali jurists. The congregant was not required to perform the prostration of forgetfulness (sujood as-sahw).