I still don't see how Migi has picked up on emotions. Migi has been the dominant factor in the fact that Shinichi should not have motions.
Is not a reason why he is gaining emotions back is because migi's cells are dying? If a small parasite is away from the host then it dies eventually, seeing that their in his blood stream they could of either:
A) Formed a second parasite clump somewhere
or
B) Died off allowing for emotions to be realeased.