What I've understood so far:

Asuma is a fallen angel. It looks like he's fallen a couple of times, resulting in his demonic form, and the angels have tried to turn him back. Each time, it seems he's picked Ibuki, or they've just failed for some reason. This is their last chance so they've taken the most drastic step, which is taking away Ibuki's memories and creating an alternate reality in hopes that Asuma will leave on his own. I'm guessing they cannot force him, he must give her up by choice. Angels are all about free will after all. The previous reality--the school where we were first introduced--seems to be something Asuma created for him and Ibuki, trapping the majority of his powers in a statue so that he wouldn't be located (power is a beacon, after all). This way he can start over with Ibuki, but in doing so, they lost their original memories of each other, possibly permanently. I am unsure as to whether or not Ibuki is an angel, but I'm almost positive that she is that or a human (I doubt she's demonic in any form) and the angels are trying to prevent further "corruption" while getting Asuma back, hence their lame tries at getting Ibuki to fall for someone else. Because this didn't work, they decided to utilize those within the school and/or temporarily give them demonic/incubi characteristics in order to seduce Ibuki. If she gives up her body to someone else, her memories of Asuma will be permanently erased and he is free to leave. Otherwise, this is just a hentai manga with a shitty plot. Lol.