I had to step away while writing this response and 3 others were posted in the mean time... I'll read those responses after posting this.
Keep in mind, the whispers in Harry's mind didn't actually stop due to the empowered steel circle around the coin. Lasciel's shadow was already firmly implanted in his noggin, and just decided to play along with his expectations. She also made it clear more than once that the circle would not block Harry from summoning the coin, either.
I'm going to have to disagree with you here.
The way you write this, the prison did nothing except motivate the Shadow to change tatics and pretend it was working. IMO, until Harry built the prison, Lasciel had an active link to her shadow in Harry, and once he built the prison, the link was sort of severed, and the Shadow was severly hampered in it's options of how to influence Harry.
If nothing else, prison actually served as is a concrete choice to reject Lasciel's influence, thus forcing her shadow to utalize much more subtle methods to gain influence over him. But I am going to hold to the idea that making this prison severly limited Lasciel's shadow's influence over him.
Jim actually wrote the Lasciel story line into DM as a last minute whim. He probably didn't work out exactly what the mechanics of the prison and shadow were until after the fact.
What twist has the series taken that even surprised you? I am constantly surprised by the twists and turns.
The whole Lasciel’s coin plot was something I instigated on the spur of the moment back in book 5, and which isn’t over yet. Lots of things happen that aren’t specifically spelled out, but which can be woven into the overstory plot that I’ve been pursuing. I expect new stuff will continue seeping out of my subconscious and prove to fit pretty smoothly into the outline.