Re: "stance of heaven (and earth)" and IS
A thought from me on heaven and earth. I believe when talked about by Ueshiba he was referring to two things in essence. Heaven related to all things spiritual and earth related to nature, the way of physical things. Thus heaven and earth. How they basically work in harmony with each other is responsibility of you to learn thus you canbe the center of such.
Thus his words spoken as a spiritual person would imply he is standing as that, the center aligned with both heaven and earth.
Physically speaking about stance I would say this. In all martial arts or let's say boxing even when I have seen good trainers they all say that correct stance is a matter of equality as far as weight distribution to feet are concerned. This usually translates as you are standing or balancing actually on your centre axis, free to move in any direction at any time. You could do this from feet wide apart or close together if you get good at it.
Thus in my view heaven and earth is about being center of, thus balanced and that concept would apply to such a physical stance also.
Just an opinion.