A well researched and thought out theory. Nicely done. There are, however, two holes I can put in it and both are somewhat subjective.
Hole One: Lasciel is a very ancient and very powerful entity. We have word from Lash that patience among the Fallen is something of an art form. If Lasciel is really miffed at this upstart mortal who thinks he can not only defy her, but can actually turn her shadow away from her, then she is going to want to make him suffer for a good long time. Lasicel is described as the Deceiver - a style that by nature involves stealth, planning, and time. She is not a thug or a brute like Magog who would be more inclined to smash an upstart mortal into dust as soon as possible. The couple years in the timeline you lay out with the whisperings seems far too short from plan inception to Harry's death to be believable as the ultimate vengeance dreamed up by a Fallen Angel.
Hole Two: In the instances you cited, other than some mild discomfort to Harry or brief despair, there was very little pain inflicted on Harry. The voices and the words spoken in your citations were not the most uplifting or positive things in the world, but they do not seem like the best suffering and personalized pain Lasciel was able to inflict on an unwitting mortal. Plus, Harry was able to overcome the words and use them for strength in the moments without too much effort.
To me, if Lasciel were really behind those voices, she would want to turn the knife in his belly for decades at least and really make those decades absolute torture for him for having the gall to think he could not only defy, but in a way defeat, one of the witnesses to Creation itself. Dresden is a wizard, and with their enhanced lifespans, she could stretch that torture out for centuries if she did it right. Nicodemus was hesitant to give Lasciel to Dresden, not only because she didn't always follow his lead, but because she was at least powerful enough to oppose him if she so chose. That would put her probably in the top 5 of the most powerful Fallen. With great power usually comes great ego, and for an affront to her like what Dresden did, the only viable solution for her would be a very long period of very awful torture. Not a few whispers of despair to convince him to kill himself a few short years of relative peace afterward. Actually, given that the words usually galvanized Harry into taking necessary actions, it almost seems like the work of Lash, taking on the role of that drill sergeant/high school football coach who likes to call people maggots and worthless and weak as motivation to prove him wrong.