Your going to have to bear with me hear… as I’m still not sure what the finished goal is for your project.
I like your 2D drawing better than I like the actual wood cap that’s being shown in the photos. The drawing elevations have the smooth curvature I’d expect to see in a climbing volute arrangement.
Whereas the wood wall cap shown in the photos doesn’t really match the climb angles that are created given the Tread Layout you show in the 2D drawings.
So the main questions I have are… What’s the main priority here?..
Do you need to model the wood cap accurately?.. and if so then what’s the next step from there?..will you then want to build a set of treads around it?.. so that the treads match the climb angles that the cap was based on?
or, are you trying to fix the 2D drawing so that those elevations match what happening in your cap form?
… and of course Rhino is also a 3D modeling program… so at what point do we switch into 3D mode here?
I was expecting to see a 3D model of the form shown in your photos, but instead was pleasantly surprised to see a nice 2D drawing of a tread arrangement that doesn’t really match what’s happening in the photo object.
Railing should follow the climb angle created by the treads… Your photo shows a shape that has a near vertical climb to it and that occurs when you have treads which create Rise, but Little, to no, Run.
[…But again, I don’t think this is really supported in the 2D Plan View and elevation drawing you’ve uploaded just now.]
Having said that, you could force this issue if you insisted on pushing down the lowest part of the volute arrangement… meaning the part sitting directly on top of the starting step. And narrowing climb angle to just the transition between tread 1 and tread 2) In doing that you create a near vertical climb at that juncture in the fittings, But, it’s at the expense of loosing the graceful natural curvature happening by considering the combination in the first three treads (…and shown in the 2D elevation drawings you made).
So there’s a few ways things could go here… but unfortunately I’m not wise enough to predict such things, and instead need the plain (dumbed down) version of the project, and/or the main goal.