I don't know if "your partner is always off balance" automatically converts the interaction from a body skill to a "relational" skill.
Ever see a wobble board or bosu board? Google it if you haven't. Its a hard plastic disc, about 2 feet in diameter, with half of a squishy, wobbly ball stuck on one side. Its designed to introduce instability to exercises. If I were to hypothetically push horizontally, hands against hands, on my identical physical twin, we'd be in stalemate. Now if I were to put this wobble board between us, the rigid, hard side against my hands, but the wobbly, half sphere for my identical twin to push on - I've got a significant advantage.
To me, I'm pushing a solid object. I don't have to stabilize. To my partner, he's got to constantly adjust himself because his hands are moving and sinking into this wobbly thing.
Our relation to each other hasn't changed - I haven't dodged, or mentally tricked him. But my partner is constantly having to stabilize and balance and can't put the full force of his body behind his push. Pushing on people who do taiji pushhands feels like this - the inexperienced use big movements of their arms, spine, and legs to keep you from "setting yourself" - but the really experienced people barely move at all. It definitely is a body skill.
If we are looking at a strong force coming in, and talking about the best way to resist that force, we are talking about one kind of thing- how to best use the body to resist strong force coming in.
If we are talking about always unbalancing someone so they cannot apply strong force to us, we are talking about anther kind of thing- How to keep people from being able to exert strong force on us.
These are different discussions, I'm up for having either/both of them, but we need to be clear about what we are discussing.