spiral staircase for loft

So then the stair would be mostly self-supporting on the center-post with your channel just stabilizing the center post and supporting the upper most 6 or so treads. I think you have a good plan, in theory.
However, the hidden studwork is a wild card. Typically,during construction, only enough fasteners are installed to do the job those walls were originally designed to be. If the studs on the rear wall were to rip out of the top plate, or more likely, the top plate let go from the end wall, somebody could get hurt pretty bad. If you have more than one teenage boy, well, they have a lot of energy.....
So while I think you have a good plan, I think it would be prudent to figure out a way to lock that top plate in place, or to prove it already is .