I think you'd be surprised by just how possible it would be to enter into a successful and good faith agreement with a warring tribe which has enslaved or massacred your people. Spanish colonists in New Mexico did so with the Comanche despite the Comanche holding hundreds of captive New Mexicans who they had kidnapped. How did this work? The New Mexicans honored their treaty, and the Comanche established a degree of personal trust with their governer, Juan Bautista de Anza. This treaty was signed in the wake of massacres on both sides, and the captives were not returned. The Comanche continued to raid and kidnap in other regions of New Spain after the treaty was signed. Part of the reason you might find something like this hard to believe is because of your own cultural attitudes about war and slavery, but these are not universal attitudes.

Your example of the Maori is perfectly analogous to what happened in Canada/ But regardless of how things turned out, why did the British honor treaties when they had no incentive to do so? How do you account for this difference in a world dominated by purely rational decision making when interacting with those outside of your culture?