Merribel isn't Yukari's past.
Merribel is Yukari's future.
Two of Yukari's eyes are in her physical body, billions of them are inside her gap. When the physical body she has now gets tired (maybe she just gets tired of it, since she can regenerate it any time she wants), she will destroy the human's soul and occupy the new body.
Could also be a backup body in case she accidentally destroys her own or Lunarians fuck it up or something.