I find it interesting that everyone assumes that lash is gone.  As far as I can tell all we have to base this on is HARRY'S thoughts that she is no longer there. Are there any outside confirming factors that she is no longer in his head and didn't use the "psychic bullet" as a ploy to retreat further into his mind and begin a second stage of temptation.  For that matter, why are you assuming that there even WAS a convergence between Lash (the shadow) and Lasciel (the fallen)? There is probably a good reason that she is known as a temptress and subtlety is her strong suit.
My counter is this: Lash is a controlled construct of Lasciel that was implemented specifically to seem to come to the "good" side altering Dresdens ability to differentiate between "good" and "evil". Lash's "conversion" was a staged event specifically targeted to soften him up. Now going into the second phase of a premeditated plan of conversion, having already offered him the carrot of a "Nice" fallen angel, she is using the stick of guilt and self-doubt to goad him into acts that he might not go along with otherwise.
If you allow that Lasciel is one smart cookie, as was stated, then why is it not beyond her to manipulate him at a distance while at the same time giving him a false sense of triumph that he "beat" a fallen angel. This just serves to give her a stronger point of attack using surprise should she ever actually decide to lock him down as Nicodemus ordered her to, that option is still on the table imho, but it will wait until the time is right for Lasciel to benefit the most from it, not another fallen angel.