Did you not read the first sentence I wrote where I said that I was aware that is almost certainly where they were going with this? Hell, I said "i think we can all see where this is going" in the comments of chapter 1.
I know that Mao is the demon lord. Even without the very on-the-nose name that just drops the "u", it's obvious that the only major differences are her age and the fact that the chapter 0 prologue showed the grown demon lord with a tanned complexion that seems to be standard for demons (and explains why it would be changed to allow her to pass as something else)
I'm just saying that in spite of them foreshadowing this from the very outset of the series, I wish that they hadn't because kid Mao is cute and the best part of this story. Sacrificing her for the grown version to be Takuma's fated partner will be a waste. Because there's no way to have the best of both worlds here. Keeping her in her kid form would make it gross and having her act exactly the same as she does now once she's grown again would be kinda creepy (albeit on brand for Japan's love of women acting much younger than they are because it's "kawaii/moe" or whatever.) They did establish that the grown version is perhaps a bit childish and naive, but it probably can't be to the same degree (I hope).