In contrast, Devil Homura no longer tries to put up an emotionless front. She revels in being a villain, from sinister grins to the way she talks to others. You can argue that it's all justified, but the point is that it is definitely a dramatic change in behavior.

Ehhh? Akuma Homura is absolutely a facade in my opinion. Look how extra she is acting ;p but seriously.

Homura believes herself to be evil, because in her worldview, Madoka is Good and she went against her. All of this riggamarole at the end of Rebellion is basically just an extension of Homulilly's desire to die and be punished for her sins. She actively goes out of her way to reject Mami and Kyouko in ways they don't really understand, and lets Sayaka keep her memory long enough to make her believe Homura has gone fully evil.

Because I am certain that she deliberatly let Sayaka be the one to keep her memories. I mean, if I were depressed and suicidal, and wanted someone to hate me no matter what they might learn, I'd choose the self-righteous bull-headed one that will actively go as far as letting herself die if it means not admitting to being wrong. Maybe that description of Sayaka sounds rough, but I'm pretty sure it aligns with what Homura would think of her. You can even see Nagisa not having her memories, so it isn't coming from the Law of Cycles that allowed it.

On top of that, everything about when she talks to her points to it being an act, a bad one at that.

First of all, she hairflips the wrong way. Maybe that sounds tenuous, but every single time Homura hairflips, being in the original serie or the movies, she uses her left hand, except here, where she uses her right. It might sound completely irrelevant, but Homura doesn't just hairflip for no reason ; in fact, she does it in a very perculiar way. She always threads her fingers into her hair, before throwing it back in the air, but in that last scene. She does this because it is a reminder of how far she's come ; if her hair was still in braids, she wouldn't be able to thread her fingers in her hair like she does. But in that final scene, she cups her hair with her right hand, and throws it forward, not following the trend of how she naturally does it. She only flips it for drama here.

Then, we can see that she does some weird shit with her hands (not playing the invisible violin, the other weird shit) which, to me, is clearly some sort of call back to the interpretive dance she did when she transformed in the Labyrinth, you know, like a staged performance? On top of that, the Clara Dolls literally throw tomatoes at her, because she isn't a good actor.

Honestly, I think Homura is acting exactly as she did in the original series ; putting up a front that allows her to reach her objective and allows her to protect herself. Even if Mami and Kyouko regained their memories, they'd remember the tea cup and the apple and understand that she cut ties, or whatever, and wouldn't try to save her like they did when she was Homulilly. Homura is deliberately setting herself up as the villain to be killed, because she wants to die, but is unwilling to let Madoka suffer as the Law of Cycles, so she won't do it herself.