Love it. Nearly every fantasy setting I've seen is either Dark Age Europe or feudal Japan, so a Polynesian/Hawaiian/Maori setting is definitely intriguing. I'd suggest looking up Lands Of Red And Gold for some hints on how to pull this off — it's about Australia rather than Polynesia, but it's the same part of the world, and you did mention that one of the main divergences involved the domestication of a native crop that didn't exist in our world (which is what Red and Gold is about), so it might provide some hints on how to do this kind of setting.
"Zealand" is not really used as a word in this setting; it was merely used for comparation, so that people less experient with polynesian languages could understand. The closest thing to New Zealand is Hinawahine, but it is closer to the concept of the Zealandia plateau being above the sea surface.
A single phrase renders Christianity a delusional cult.

Note: I so far have had no contact with fantasy works set on a polynesian setting, aside maybe from Pterry's Nation, although the fictional culture's mythology seems to be barely based on polynesian mythology, which gives me some room for originality.
Basically, I'm designing a fantasy story set on a Polynesian setting, mostly based on the Maori and Hawaii, with influences from Tonga and Madagascar (as it seems that the native people of Madagascar share a common ancestry with the Lapita, which produced the polynesians).
Because the polynesians were already on their way to produce full civilasations (Hawaii, Tonga, Rapa Nui), I decided to make the process go faster by introducing polynesians to larger and richer landmasses than the average Pacific island; the main setting is the insular continent of Zealandia, a landmass that once united New Zealand to New Caledonia, and that eventually most of it was sunken beneath the waves in our world. In this world, most of the continent is still above the sea (at worst being an archipelago of large islands), allowing more room to civilasations to develop. The presence of several sweet potatoe like plants also helps, as well as divine intervention (being fantasy, after all).
So the polynesians of this setting began in Hawaiki, the place that all polynesian mythologies share in common as their home land and underworld, that followed the same path as Easter Island, becoming a sterile land. However, thanks to the gods the natives found Zealandia, which to them became known as Hinawahine ("grey haired woman"). Meanwhile, others ended up in Tepitootehenua, a stand in for Easter Island, and now the two nations are at war. Other nations also exist, so far unnamed.
So, is this original enough, or have I accidently copied someone else's ideas?