Wheelbuilding / Wheel Building / Wheel build help

Isn't it more the case that because there are a lot of parts that are kinematically constrained (though in complicated ways), the entire system (once you take into account the spokes) becomes massively overdetermined, which is what makes the exact load calculations so difficult?

I'd say so, yes. So it's easier, rather than trying to model the individual components of the wheel, to work on the basis that you have a decelerative force at the contact patch and this is being resolved by the wheel as a whole to the axles, which are the only parts of the wheel system rigidly attached to the frame and forks. The only things connecting the axle and the contact patch is the spokes, so for the same decelerative effect at the contact patch and the same number of spokes the average spoke loading must be roughly the same.