Considering Lex Luthor's confirmed involvement in the film, being brought to life by Jesse Eisenberg, this becomes suddenly more likely. Mercy Graves was introduced into the DC universe via Superman: The Animated series as a bodyguard to Lex Luthor, and became popular enough that she was incorporated into the comics. She often seen with short hair and a chauffeurs uniform, and her appearance in the Superman cartoon could be seen as quite similar to that of Jena Malone. In addition to being Luthor's right-hand woman, she is keenly intelligent, fearsomely strong, and is proficient in all types of hand-to-hand combat, kickboxing in particular. All of this could potentially lead back to the original leak from October. The scene being filmed during that time was a 'physical confrontation between Bruce Wayne and Lex Luthor' at a formal function. Perhaps Jena Malone steps in as Mercy to protect her boss, and perhaps her getting physically involved in a fight scene led to confusion about there being a female Robin in the film? And even if Malone definitely was playing Robin...