A good article. Thanks. It's given me some things to think about and build upon what I've already been thinking about.
Just to touch upon this part,
"If the defender is "blending" with the energy of the attacker, and the attacker is blending with the energy of the defender, who controls the interaction?"
I guess this ties in with the theory of leading in aikido. Where you lead but you really aren't leading. At the point where you are defender and using all your concepts and moving along the seam, you are somewhat leading (or guiding) the attacker. This is because the attacker is going somewhere he/she didn't specifically want to go. But you aren't really leading the attacker because you, as defender, aren't taking him/her where you specifically want him/her to go.
But, when the situation is like the quoted above where the "blending" is done by both ...
I don't know. Maybe someday.