Very interesting thoughts you share. However:
You talk about there being no single plain for pronation/supination and talk about the ankle joint. What about the subtalar joint? This is a three faceted complex (although there is debate about whether one particular facet is the only true subtalar joint). The overall motion of this joint is one of pronation and supination. Yes, this is a triplanar action, but that doesn't necessarily mean there is no axis about which it turns.
For simplicity, we speak in terms of three cardinal body planes, and motion that occur in or through thoise planes. So when we speak about a joint motion, the axis of which crosses all those planes, we speak of it in terms of the constituent parts of that motion eg inversion in the frontal plane, abduction in the transverse plane, and dorsiflexion in the sagittal plane. We say this because we do not have any planes other than the ones of the x,y,and z axis'. The overall, combined total of the sums of those motions, we call pronation.
When there is pronation about the subtalar joint axis, the foot moves smoothly, it does not evert then abduct, then dorsiflex, it does one movement.
Good to talk about such wonderful things as foot mechanics!