Kash, I dont think Milind will live with a possessed Prachi without suspecting anything because not only he is aware of the bhooth. Bua is aware of it also and has warned every body about it. So unless Ayesha kills every body, she is not going to get away with it. Moreover, her basic qualities are going to surface sooner than later which is going to make people suspicious of her again. So no, I dont think Milind will live with Ayesha, even if she is in Prachi's body.
But I cannot rule out the possibility of Ayesha killing Prachi. If she does, I am sure Milind will kill himself in grief. If Ayesha gets into Prachi's body in the meantime, then she is trapped in the living world while Milind and Prachi will meet up in the next world and live happily every after.
I would not rule out this weird scenario for two reasons:
1.The show title - Day of the doom/judgement
2.The show theme - Changing relationships, every lasting love. So far they have experimented with evey possible change in Michi relationship. Maybe they may end it with this saying that even a change from mortal status to immortal status could not seperate Michi.
This is a horrible way to end the show but I would not put anything past these guys. I personally want a five year leap next week where they show that Bhooth is finally gone for good and some good Michi moments before they end the show.