Unless I'm interpreting it incorrectly, Sigman's balloon man model doesn't really care about the joints... so for all intents and purposes, in the IS model there is no fulcrum... only control unit(s) which is/are the dantien(s). The absence/elimination of fulcrums in the IS models is what enables "jin everywhere all the time" (what you're calling groundpath in all directions). This is presumably also why posture and alignment ultimately don't actually matter in the forming and maintaining of groundpath, since a balloon man by definition has nothing to align.
So the basic idea of the "balloon man" model is that the whole body is 'pressurized'. This means that all of the joints are equally filled with a pressure, which enables the muscles of a stressed joint to relax instead of work, because the pressure is taking the load. Is this correct?
My first question with this is, what's making the 'pressure'? Is it air pressure? Is it hydraulic type pressure?
What is inside of the body that can be 'pressurized' evenly throughout? Is it blood or some other body fluid?
What is 'pressurizing' the medium that we are adding pressure to? Is it the muscles of the core- the dantien?