A lot of fans are treating it as a fait accompli. This isn't like when we all thought Sayaka was Walpurgis Night - Nagisa increasingly is looking like one of the human personifications of Charlotte you see in fanart. It's just her design is more subtle about it.
It could be an elaborate red herring...but we did get magical girls = witches and Homura = time traveller, so maybe we should trust our instincts here.
(Besides her VA is Kana Asumi. Who else would you have voice human Charlotte?)
this is Urobutcher we are talking about after all though.
The joke about certain fiction authors certainly deserves updating in his honor. You know: "Joss Whedon, George R. R. Martin, and Urobochi Gen walk into a bar. Everyone you love dies."