Another thought that I've had recently, though I dare not put this in Hazou's head:
Even if Rock is planning to vassalize Leaf, and pulls it off, that doesn't exactly mean Rock wins in the end. Great Britain was unified by Scotland, after all, and it didn't take long before England was the main seat of power and culture, and the same might happen here. Leaf has a larger population and economy than Rock, so it's perfectly plausible that, say, a Rock/Leaf dual-Kageship would over time lean more and more towards Leaf until Leaf is de facto the dominant partner economically and culturally.
Of course, Hazou saying that would be treasonous or something probably, but OOC if Rock is going for vassalization then Leaf's got a good shot at absorbing Rock in the end instead of the other way around.