What is the ruling on congregational supplication, reciting Al-Fatihah, and performing Salatul Fatih at the grave after burial? And was any of that reported from the Prophet, may Allah bless him and grant him peace?

Supplicating for the deceased after burial is a Sunnah. One must seek forgiveness for the deceased and ask Allah for their steadfastness, based on the Prophet's (peace and blessings be upon him) hadith: "Seek forgiveness for your brother and ask for steadfastness for him, for indeed he is now being questioned." As for reciting Al-Fatihah or Surah Yasin after burial, or repeating Salat al-Fatih, these are newly invented innovations that were not authentically reported from the Prophet (peace and blessings be upon him). The best guidance is the guidance of Muhammad (peace and blessings be upon him). It is incumbent upon seekers of knowledge to clarify the Sunnah to people with wisdom and good admonition.