Could the Phoenix be confusing their elemental imbalance problem by associating two separate issues (the tsunami and the elemental imbalance)? Maybe that's why they can't get a good handle on the issue. Perhaps the tsunami is a result (either deliberately or inadvertently) of the Mantis using the kami in this way off the Crane coast to raid them. At least that's where my mind went while reading the story.
Feels to me that by weakening the Crane's naval presence (and allying with the Crab) the Mantis are trying to set themselves up for naval superiority along the coast. Right now if they wanted to invade the Crane coast then Lion would not come to Crane's aid and Crab also would not aid the Crane. The Crane ally the Unicorn isn't positioned well at all to assist them in defending their coast. The Phoenix seem paralyzed by their own problems. Seems like the right time to strike for Mantis if that's their plan. Crane are in a pretty bad spot.