Hmm, I'm not sure that AltCalliope needs to possess Jade to have narrative control, at least to some extent. She is the black hole itself, and it seems reasonable to me that she could have some control over it for that reason alone. Even if she does need Jade, she's able to exert some influence without having possessed her: in Meat Epilogue Four, she's able to fight against Dirk's narration to influence Jade to fall into the black hole. Once she does, she's able to possess the teen Jade, and that allows her to also possess Meat Jade, giving her control over Meat as well.

In general, though, I think AltCalliope has always been the "author" of Candy, and that her reason to possess Jade was not to gain control over Candy, but rather to gain the ability to fight against Dirk, both by narrating Meat and attempting to stop his plans, and also by using her new physical body to confront him. And also eat Lord English. As Alt-Calliope says, the work will not be finished until she is able to "deal with the prince" herself.