If the whispering shadow was Lash, then Lash didn't have Harry killed--Harry had himself killed.
Remember, the seven words didn't include, "Then kill yourself." They were a guilt trip, probably meant to make Harry seek power--if Lash is whispering, then she'd have preferred said power be Lasciel, but another source of power (that would heal Harry) would also help ensure Lash's and Harry's survival.
But we know-- or, at least, Uriel has led Harry to believe-- that the Whisperer intended for its words to convince Harry to kill himself. According to what Uriel said, the Whisperer knew exactly how Harry would react, and it said what it did, when it did, to trigger a suicidal reaction. That was supposedly how the Whisperer cheated, and the fact that it cheated was the reason that Harry was allowed to return to Earth in the first place. This chain of events seems to disprove the "Lash screwed up" theory.
The shadow is lasciel , i believe in a new host. Lash is the parasite. This has been i believe confirmed by jim. (not the new host part). Also let slip by Jim is there is a conection between maggie sr and the black court, and that they will be showing up soon. As to lash herslef, she knows what the black court is up too.. Not nec the outsiders. Rember her last words in wn? Shes taking a nap until after jim does his big blampire amry reveal
As Serack quoted, it's been unofficially confirmed that Lash is the Parasite. It has
not been confirmed that Lasciel was the Whispering Fallen. That's just the most obvious place where Lasciel could have shown up in
Ghost Story.
Myself, I always feel like that's too easy a guess to make for Jim to bother hiding it at the end of GS. It feels to me like the guess we're supposed to make, and then we find out later that the truth was something sneakier and more surprising.