Should I say "Amen" after reciting Al-Fatihah silently while the Imam is reciting the Surah, knowing that saying "Amen" is commendable and listening attentively is obligatory? And does the prayer of one who says a commendable remembrance, such as the opening supplication or "Amen," invalidate while the Imam is reciting the Surah after Al-Fatihah?

Saying "Ameen" (Amin) is recommended after reciting Al-Fatiha, both within and outside of prayer. It is recommended for the worshiper to say "Ameen" after Al-Fatiha, even if the Imam is reciting a Surah (definition), because it does not prevent listening attentively. The prayer is not invalidated by reciting the opening supplication (Du'a Al-Istiftah) while the Imam is reciting a Surah, though scholars have differed on its permissibility in this specific situation.