You have the concept correct; however:
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But I'm still wondering why you won't use the sprites as hand drawn art together with the stencil shadows
What do you mean by this? How is the stencil shadow going to interact with a bitmap and contour to it's details? It can't which is why the invisible collision hull must be in place and anyplace the player's shadow can touch much closely match the 2D sections.
Here are a couple of quotes from a very famous adventure game artist/designer working on the adventure game A Vampyre Story. They're using the exact same technique I'm stealing from them. 
Perhaps, his explanation will be better. There are two of them taken from two different posts:
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So AVS will be in 3d BUT,,,,not the way you think. I am a big fan of the early Disney movies, and one Oscar winning short in particular really influenced me. It was called The Old Mill. http://disney.go.com/vault/archives/movies/oldmill/oldmill.html It was mood film, meaning it didn’t really have plot, but for good reason. It was a test film for a new technique called multiplane camera. So we are going to do is make a virtual multiplane camera using your PC’s 3d card. The backgrounds will be painted in 2d but placed on 3d planes. This will allow us to keep the backgrounds looking 2d and hand painted, but we get to create the illusion of depth and perspective.
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We draw the backgrounds on paper then scan that in and paint the set layers in Photoshop, with each plane on its own separate PSD layer. Then we save the separate layers as TGAs (Targa files) and turn them into textures, along with their alpha channels (transparent matted areas) in Maya, and then UV map the layers on to flat parallel polygons in Maya. Export that file into our engine, test it make sure it works. Then add particle affects and UV scrolling areas. Add the dynamic lights for the sets and 3d Character models. Import a separate set of geometry for the walk boxes, test that out. Then add all the interactive objects like doors and buttons and things like that. We use Maya to build the character meshes, rig them and to animate them. Nothing really exotic in this process. They only thing unusual is that we aren’t building 3d models for use as our sets, just 2d planes.
Go here: http://www.adventuregamers.com/screenshot.php?id=6823 to see some screens from this game. The bottom 8 are the new ones. All of those screens are broken up into layers (the parallel planes), so it's a 2D game but because of these layers, it will have depth. It's like parallax scrolling in the old 16-bit days, but a bit more advanced in the execution because of the use of a full, realtime engine. This will allow for zooming in/out and moving the camera into the scene or out as necessary. The technique will be more apparent in exterior scenes. Anyway, I will need my shadows to interact with my backgrounds or else it just won't be as convincing an effect.
