The Justice Department is claiming, in a little-noticed court filing, that a federal agent had the right to impersonate a young woman online by creating a Facebook page in her name without her knowledge. Government lawyers also are defending the agent’s right to scour the woman’s seized cellphone and to post photographs — including racy pictures of her and even one of her young son and niece — to the phony social media account, which the agent was using to communicate with suspected criminals. The woman, Sondra Arquiett, who then went by the name Sondra Prince, first learned her identity...