Is the recitation of Surat Al-Fatihah obligatory in every rak'ah, or is it sufficient to recite it in one rak'ah along with reciting whatever is easy from other surahs in the remaining rak'ahs? And does the prayer become invalid by doing so?

Reciting Al-Fatiha is obligatory in every rak'ah (unit of prayer) according to the majority of scholars, including Malik, Al-Awza'i, and Al-Shafi'i. This is also the correct opinion within the Hanbali school of thought. Consequently, prayers in which Al-Fatiha was omitted must be made up. However, Sheikh Al-Islam Ibn Taymiyyah held the view that whoever omits a condition or a pillar due to ignorance of its obligation is not required to repeat the prayer. This opinion is strong and may be adopted.