Is reciting Al-Fatihah collectively at a mourning gathering, after supplicating for the deceased, considered a Bidʻah Ḥasanah (good innovation), and what is its legal ruling if it is not?

The origin of offering condolences is legislated, but the early generations of the Ummah did not specify a particular manner for it or dedicate a specific surah to be recited. Gathering at the house of the deceased for condolences is a reprehensible innovation (bid'ah dalalah). The rituals mentioned in the question have no basis in Islamic law and are an innovation, because the Prophet (peace be upon him) said: "Every innovation is misguidance." There is no such thing as a "good innovation" in religion; every innovation is misguidance. Whoever deems something good has legislated it. Imam Malik said that whoever introduces a "good innovation" has claimed that the Prophet betrayed the message, because the religion has been perfected.