I know, it's all just speculations, no one can prove anything.
But it could also be that Lash was in that part of Harry's brain that didn't take the damage. She could have been sealed off because another part of the brain was damaged, which made her unable to contact her host. Maybe she cut off the relatively new Lash-part of herself, so that only the original shadow remained in the sealed-off part of the brain. What is left, isn't Lash anymore. It is Lasciel's shadow all over again. The Lash-part was involved in creating Bonea.
Now the brain is healed and the shadow slowly gains access again. Or even has had access for a long time again, but chooses to stay hidden, like you said (g33k).
The sealed off but not damaged part: I've known a patient who had had a stroke and they said, she was blind because of it. But we found out, that she could still see, but the part of her brain, which made her access the information about the things she sees, was damaged, so she saw everything, but couldn't connect the information her eyes took in with the part in her brain, which made her understand what she saw.
I hope I could make that clear. English is not my first language.