Is it permissible for a lone congregant to stand in the same row as the Imam with social distancing, or should he stand behind him due to the social distancing requirements of Corona, if he is unable to stand next to the Imam or slightly behind him?

The lone congregant stands to the right of the Imam, aligned with him. The purpose of alignment and proximity is to achieve the congregational prayer. If harm or transmission of illness is feared from alignment and proximity, then distancing is permissible, based on Allah's saying: (Allah intends for you ease and does not intend for you hardship) Al-Baqarah/185, and His saying: (Allah does not intend to make any difficulty for you) Al-Ma'idah/6. If alignment is impossible due to necessity, the single congregant stands to the right of the Imam in the same row, even if distant from him, because necessity is judged by its extent. The prescribed action for a single congregant is to stand in the Imam's row and not to pray alone behind the row.