Is it obligatory to recite the opening supplication (Du'a al-Istiftah) and Surat Al-Fatiha when joining a congregational prayer while the Imam is reciting a Surah from the Quran?

Reciting the opening supplication (Du'a al-Istiftah) is not recommended for someone who joins congregational prayer while the Imam is reciting, because the opening supplication is a sunnah (recommended) act, while listening attentively to the recitation is obligatory. As for reciting Al-Fatihah behind the Imam in a loud prayer, the preponderant opinion is that of the majority (Jumhur) who hold that one should not recite it, based on Allah's saying: "And when the Qur'an is recited, then listen to it and pay attention that you may receive mercy." Therefore, it is obligatory to listen attentively to the Imam's recitation in a loud prayer unless he pauses or his voice does not reach you.