Great Antosha
1) In the moments you described, I don't see anything critically important, but since you've decided to change some things yourself, with your permission, I can add your changes to the mod I've uploaded.
2) I'm hearing about this Campaign for the first time. If I'm still alive and well in ten years, then we can think about something like that.
3) Keep in mind that this race is already imbalanced compared to other races in the fifth installment, so all such changes need to be implemented carefully. Simply trying to "make it like in the sixth game" is not reasonable. Whether the way creatures look in the fifth game, which is somewhat different in the sixth, is a matter of debate. But, compared to the original size of the Kappa, for example (I already have a model transferred from the seventh game, and it's actually too small for the fifth game), they clearly belong to the first tier, compared to the Sharks. I think we should either remove the water immunity from the Kappa or add an aura of vulnerability to air to all of them, from the elemental Gargoyles. And the Kirin need to have their speed reduced; they can now run across the battlefield from one end to the other even without any bonuses.
Added 40 seconds ago
Everything unpacks normally for me. Try using 7zip, which I used to pack the archive.
3). I'm saying that the Kappa had less health, and here they can be smaller in size, but they had more armored defense and a faster initiative. Ideally, they should have a new ability in the form of just 50% Water Resistance, but if that's not possible, then at least leave them with Water Immunity. But I only see one reason not to do that, which is not related to balance - Inferno doesn't have any creatures that are immune to Fire... And it's not just Air Vulnerability - it's an AURA of vulnerability, which would only be suitable for Water Spirits, Kappa, and Kirin, around which water splashes. And for the Ice Mizu-Kami, an Aura of vulnerability to Fire.
Are you sure about using that sub-mod in your mod, which restores the costs to what they were in the Forest Union? It might not be enough for the Kensei and Frenzied Kensei to cost 1400...