I think that there is some sort of mechanism that allows a vampire to 'unbind' their bride and restore them back to normal, but that this mechanism has a high cost. I think it forces the vampire to immediately revert to the Chrysalis state and lose all memories. I also believe that Iris has done this before (with Mizuki) and that Shion knows about it. I believe that Shion is keeping this a secret intentionally because she doesn't approve of using this mechanism - it defeats the purpose of being immortal if you're returning to square one just as your adult life begins every time.
There's a few reasons I feel like this is right, but they're mostly meta-narrative reasons. Basically, the ending based on the state as is (Sara dying, Iris alone and forced to pick another bride to die) is too sad. Some twist will occur to at least slightly improve the mood before the ending. Additionally, I feel like the author put an overdue emphasis on pointing out repeatedly that vampires "can't give back" and "can only steal". Iris asks herself for an entire page why she can't help people with her ability. It would be fulfilling in a narrative sense if there was a way to do it. Lastly, I've established why a twist must occur, but let me elaborate on why -this- twist will probably occur. There's a few reasons (again). First, Iris' fate of repeatedly killing people is too out of character for her. I feel like it'd be in character for her to commit suicide rather than kill a woman a year forever. Second, Iris' research can't pan out with an entirely new discovery. It doesn't make sense why a 16-year-old girl would be able to succeed in 6 months where the world's most influential people in an extremely powerful organization couldn't. It would make sense if she discovered some secret ritual or mechanism that they were hiding through her research, though (because they want vampires to stay alive to make themselves more powerful). Finally, there needs to be some way for Sara and Iris to be 'together forever' to completely fulfill the premise. I think that this repeat cycle of Iris being forever young and Sara having Iris grow to love her each time would count (at least until Sara dies).
Alternatively, it's possible that this reversion occurs for both vampire and bride, and Mizuki is Sara. I'd say that's pretty unlikely, but it is still possible.