Before theres any thought of attacking Gallia, the crises within Kusari would have to be resolved and a definite leader put in place. Because last thing Kusari wants is to go to war and then half way through it, its whole economy collapses, and infighting crops up again. And that is assuming no other issues crop up in the near future, so going to wars definitely a good stage off - I'm guessing anyway.
Especially if you get some of Kusari defecting to Gallia, although with the Kusari honour system i doubt that's gona happen to any huge degree, but still something to watch out for.
But yeah i agree that with their Emperor fighting over in Bretonia a good portion of Kusari should feel honour bound to move in and help, here hoping anyway or i'm gona have to learn the phrase "I surrender and volunteer for mutiny" pretty quickly.