Is it permissible to perform congregational prayer at home if one fears showing off (riyaa'), or has heard accusations of the imam practicing sorcery?

Do not abandon prayer in the mosque based on people's unproven statements about the Imam. The default assumption for a Muslim is innocence, and for an Imam, that he is among the best of people. It is not permissible to accuse him of sorcery without clear evidence. Among the deceptions of Iblis is to make a worshipper abandon prayer in the mosque out of fear of showing off. One is protected from sorcery by strengthening faith, diligently observing adhkar (remembrances), supplication (dua), and abundant remembrance of Allah.