What is the ruling regarding the Imam not enabling the congregants to recite the opening supplication (Du'a al-Istiftah) and the supplication before the Taslim in a loud prayer, and when should the follower (Ma'mum) recite Al-Fatihah if the Imam does not pause between verses or at the end of Al-Fatihah?

The worshipper following the Imam must recite Al-Fatiha in every rak'ah, whether the prayer is silent or audible. The worshipper should recite it during the Imam's pauses or while the Imam is reciting. As for the opening supplication (Istiftah), it is a Sunnah. Scholars have differed regarding its ruling for the worshipper when the Imam is reciting audibly: The Shafi'is and some Hanbalis are of the opinion that the worshipper should recite it unless there is a fear of missing Al-Fatiha, in which case reciting Al-Fatiha takes precedence. Others, like the eminent scholar Ibn Uthaymeen, favored the view that the worshipper should not recite the opening supplication while the Imam is reciting, but rather listen attentively to the Imam and only recite Al-Fatiha, because the opening supplication is a Sunnah, while listening attentively is obligatory.