If all muscles were completely relaxed what would move the body? I think this might be the formation of a better Hypothesis 3.
Well, if all muscles were completely relaxed, I wouldn't worry about moving, but about not falling on my face.
So I have to agree with Dave: muscles are needed to move the body. The problem with all this IP/IS stuff, though, is that it does not feel like the muscles are used to move the body. You feel more like your body is some elastic entity, with the dantien controlling the tensions in that entity to generate movement.
Or to put it more broadly, the problem with muscles w.r.t. IS/IP is that what the body does physiologically (Not that I know anyone that fully grasps the physiological side of IP/IS, btw.) is very different from what it feels like and from what it takes to get people to move in a IP/IS way.
Which is not that weird, I have heard a similar thing about the pose running technique: it requires you to lean forward from the ankles, but if you tell people to lean forward from the ankles, they will do all sorts of things, except lean forward from the ankles...
Which is the same points as ChrisMoses was making, I think. And sure, there's a way "to intellectually understand what is happening, even if we can't do it, or feel it", but then I would feel more comfortable talking about a balloon man than about muscles, which may not pass your standards of intellectual rigour.