Is it valid to pray in congregation if the Quran is being recited in the mosque? And is it permissible to join someone who is praying alone to become an imam?

Raising one's voice in the mosque, even by reciting the Quran, is not permissible if it disturbs those who are praying, due to the Prophet's (peace be upon him) saying: "And let not some of you raise your voices over others with the Quran," and his saying: "So let not some of you harm others, and let not some raise their voices over others in recitation." Congregational prayer with a raised voice is permissible and valid, but the sin falls upon the one who raises his voice. Reading the Quran aloud in the mosque due to someone's death was not known among the Salaf. And it is permissible and desirable to follow an individual in prayer.