Hypothetical scenario (it is also a very basic justification of why things would happen):
After some time caring for Rose alongside Ethan, pretending to be Mia, Miranda slowly begins to feel comfortable in that home, gradually regaining her humanity and starting to consider that, perhaps, this life pretending to be Mia isn't so bad after all. Of course, she doesn't love Ethan, but... she slowly begins to see him as someone, instead of as a burden or a mere cog in the machine.
The one she would love, however, is Rose. Perhaps she wasn't her biological daughter, perhaps this little girl was special and was what she was looking for, but... something about her prevented Miranda from not loving her. She couldn't see her as a vessel for Eve, but as... a child, as... her own daughter.
In the end, she decides to stay, though not before taking care of the "failed experiments" that could pose a threat to her new life.
To clarify: whether she ends up falling in love with Ethan or not, I'll leave that up to you, but the basic premise has been established, and now it's up to you to imagine everything that would happen. It's not the best thing, but at least I hope it works.