How sound is what was mentioned, that whoever doubts the recitation of Al-Fatihah after bowing is required to repeat it and what comes after it? Is this present in other madhhabs? And why did you only adopt the opinion of the Shafi'is? And must the prayers in which this opinion was applied be repeated?

In matters of ijtihad (independent reasoning), one who chooses between two opinions is not to be blamed. If one inclines towards preferring one opinion or follows a trusted scholar, they have done well. The muftis (those who issue fatwas) whose fatwas you heard are not at fault if they issued fatwas based on what they deemed correct, nor is one who follows them at fault if they are qualified to be followed.

As for this particular issue, we have preferred the view of the Shafi'i school regarding the prayer not being invalidated by doubt concerning the omission of letters or verses, in order to alleviate hardship, and considering the apparent fact that the doubted part was indeed recited. The Maliki school is similar to the Shafi'i school, or even more lenient, as they believe that one who forgets a verse from Al-Fatihah can rectify it with the prostration of forgetfulness (sujud al-sahw), and doubt takes precedence.

It is stated in Mawahib al-Jalil: "If a grammatical error occurs inadvertently during prayer, it does not invalidate the prayer, whether it is in Al-Fatihah or elsewhere, and whether it changes the meaning or not; because it is like someone who spoke inadvertently, or omitted one, two, or three words inadvertently, and this does not invalidate it."

As for the Hanbalis, we have not found a specific text from them on this issue. Their statement that whoever doubts a pillar of prayer is like one who omitted it is not sufficient to generalize the ruling to our specific matter, because the Shafi'is share this principle with them.

Therefore, one who acts upon an opinion they deem correct or follows a trusted scholar is not to be blamed. Re-performing the prayer is not obligatory for one who returned to recite the doubted part of Al-Fatihah after ruku' (bowing) if they have an interpretation that allows for such an action.