For the record:

Yanini does not exist. (Turare, Haraiki, and Amanu -- the three islands in the SI cluster that the tribes draw their names from -- do.) The Society Islands became the setting because -- well, honestly? Because of what Alex says in Episode #13: just about every season, Jeff talks about building a new society. It's been one of the show's self-presented arc themes since the first season, and sticking a cast in a place with the word 'society' in the name just seemed like a decent idea at the time. It also gave me a tropical setting, one that the show hadn't been to before even if they'd been extremely nearby, and the chance for the cameras to get those all-important swimsuit shots.

The island is an artificial construct all the way through. The chaos of the plant life was probably meant to invoke a sub-theme of sorts: elements from everywhere forced to co-exist without tearing each other apart -- at least, not just yet... (I'm once again trying to guess what my subconscious was up to here. Wish me luck. But Jeff says as much in Episode #1.) And it also saved me from having to go with the usual tropical plant background and a coconut-heavy diet. Just about any plant from anywhere could appear on the island, which gave me a degree of freedom in mood-setting: Turare's large oaks bracing the shelter and shady leaf canopy stand in contrast to Haraiki's more tropical clearing, lack of support, and exposure to the open sky.

And yes, the black sand vs. the white sand was pretty blatant.
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The Society Islands cluster is just south of Marquesas and east of the Cook Islands, which would give the show a few seasons in the same general area. Well, at least it's warm...
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The name for the Cliffs Of Insanity was borrowed from The Princess Bride, with thanks.
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Haledon is real. It's in northern New Jersey, just above Patterson going up the hill, and on the opposite side from Wayne. The city itself is a little below William Paterson College, which Wayne claims for itself. (The woods around the college are as Alex finds them -- and there's a major cliff in front of the junior/senior dorms. Do not stumble through that parking lot when drunk -- but on an exceptionally clear day, you can see Manhattan.) The economic situation of the area is pretty much as described: you can see the incomes increase block by block as you move deeper into Wayne. And no one walks that hill if they have a car.
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Alex's apartment building is based on a couple of absolute ratholes I had the mispleasure of encountering when I originally went hunting. ('Don't worry about the dead cockroaches in the bathtub. They're dead, aren't they?') And that's an insult to rats.