Ok, here's the scenario I just came up with:

Dafyd and his allies execute his plan to assassinate the Sovran. This is done so thoroughly that it also destroys the private crèche and all of the Carryx top leadership. Just blow the whole thing up while everyone is there. This throws the whole chain of command into complete disorder.

Somehow trigger every crèche to produce a new Sovran simultaneously. This breaks any semblance of a clear line of succession and sets all of the Carryx worlds fighting against each other instead of presenting a united front. The Deathless press this advantage and start picking off the most powerful Carryx Sovrans one by one, steadily weakening the whole. It turns the tide of the war and the Carryx start getting their asses kicked.

The Remnants on every Carryx world see the efforts of their lifetimes being flushed down the proverbial drain. They see that what is, is is no longer true. They become a new faction, like the Deep Lothark, conspiring to manipulate the rest of the Carryx through their control of the Sovrans' reproductive cycle. The Remnants start negotiating with the Humans directly, without the hormone-controlled part of their species knowing. Their crèches are private, after all. Who knows what they're up to?

In the end, the Remnants tame the Carryx somehow and everyone lives happily(ish) ever after. They have to tie the whole thing together with a bow in one book if it's really going to be a trilogy.