And a liberal democracy that prizes multiculturalism, I absolutely agree they are a good fit. You get all that human capital and goodwill too, it's good business.
Perhaps, perhaps not. In Hawaii there is actually a lot of tension between the Kanaka Maoli (Native Hawaiians), the Marshallese, and Micronesians. I am not keen as to exactly why this is, but I think it underscores the importance of providing more resources for both the Pacific Islanders already living receiving limited assistance, and the new arrivals. I think the government shouldn't just assume they'll get along because of shared cultural heritage.
I don't know if there's a word for it, but pretty much every nation has at least one other nation that it traditionally looks down upon, usually one that to outsiders seems quite similar and which often have in fact strong ties between them. France and England are a classic example, though maybe a little odd in that it's mutual. (I suppose Argentina and Chile are similar in that regard--each thinks the other is categorically beneath them?) Sometimes there's a clear regional hierarchy, like U.S. -> Mexico -> Guatemala even though it's not necessarily obvious to all involved--U.S. has traditionally looked down on Mexico but doesn't have any sense of a hierarchy beyond that, whereas Mexicans might not care less what people in the U.S. think but certainly feel they're better than Guatemalans. Sometimes it's a non-transitive circle. It can be very complex, and it doesn't have to make any sense, though often it's a simple reflection of comparative wealth or prestige. But it's a universally understood phenomenon. There's the famous quote by Jean-Jacques Dessalines, leader of the Haitian Revolution, who circa 1804 called the Polish "the White Negroes of Europe", exempting them from the murderous extirpation of all white-skinned people in the newly independent state. Fast forward to the 21st century and British prejudice against the Polish (and specifically Polish immigrant laborers) is a major impetus for Brexit.
Anyhow, I've learned to be careful about equivocating nations or cultures. Saying something like, "country X seemed just as nice as Y" might rub someone from X or Y the wrong way.
It's all thinly disguised racism, of course. But it does pose an interesting woke paradox: what if all a nation has left is its prejudice against another nation?
I think it's actually that human cultures are infinitely nested, with every group containing subgroups with some level of disdain towards each other. Conflicts on a higher level often seem to "override" conflicts on the lower levels (although I don't know how often this actually is true). For example: I'm from Finland, and if our team plays hockey against Sweden it's as if there's a war against them or something. However, if Sweden plays against Canada you'll still see people cheer for Sweden over here.
I believe this reveals the existence of a tree-like hierarchy (very loose) of human cultures, and I also believe it goes way lower than just the country level (all the way down to individual people). xkcd also said it a long time ago: https://xkcd.com/1095/
I agree, national enmity is a manifestation of a broader phenomenon. Part of that broader phenomenon is that identity is typically defined, at least in part, in opposition to another group. And identities can nest, intersect, etc.
In context of island erosion, this poses the question of whether a nation without any territory can exist purely in opposition to another similarly situated nation (i.e. both sharing a positive history that is indistinguishable within the context of a broader society to which they've emigrated). And that creates an interesting ethical dilemma if the goal is to preserve national identity, especially one that has been "victimized" by [perpetrators of] global warming.