What is the ruling on the validity of the prayer and the prayer of the congregants in the event that Al-Fatiha is recited in the second rak'ah, and then one bows for rukoo' by 20% without saying "Allahu Akbar," then remembers not having recited the Surah and returns to recite it, then bows for rukoo' without performing the prostration of forgetfulness (sujood al-sahw)?

The opportunity to recite the Surah (definition) is lost by placing the hands on the knees, or by bending for the bowing of the next rak'ah, even if one has not achieved ṭuma’nīnah therein. However, bending that has not reached the point of placing the hands on the knees does not cause the loss of the opportunity to recite the Surah, and nothing is required because of it; the prayer remains valid.