Why doesn't Eve, since she loves Hazuki so much, simply decide to grow old with her in the book world - preferably picking things up from the moment she became 16? Eve is supposed to be pretty much all-powerful, so she could have arranged something like that easily, especially since age doesn't count for immortal beings.
Related to the previous point: what is the use of Eve giving Hazuki one night with her, only to wipe her mind afterward? Quite a pointless exercise, especially with those love letters that don't belong to anyone anymore in the new world.
The answer to both is that Eve is just a massive bitch who doesn't give two shakes about anyone but herself.
That's not completely true, since she basically gives Gargantua what he wants. She also plans to return to Hazuki as her child, so she does care to be with her... but why do it in such a redundantly roundabout fashion?
This is actually a trope which I will write into TV Tropes one day, as soon as I gather enough material on this. Basically, Japanese lesbians are Blessed with Suck, since society expects Japanese girls to a) grow out of lesbian relationships and b) later marry a man, in order to c) bear children. This is so strongly in the heads of the Japanese men, anyway, that even Yuri-series's have this trope. Yami Bou had the "I can't be your lesbian lover but I will return as your child born out of a perfectly happy heterosexual relationship to you" ending. Yuri-series's made thirty years ago have done it, making it the oldest one in the book. Times are changing, but only slowly, because most anime and manga are if not made then still published by men. Thankfully, women's liberation in Japan is actually a big step further than still depicted in media.
Also, Yamibou has several different endings, none of which go together. One of them is an open-ended but happy (and legitimately lesbian) ending; one is completely miserable; one is...thisone. They're smushed together so that they seem sequential, which is bullshit in and of itself but less ridiculously disappointing an ending than it would be otherwise.