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1) Yes.
2) They follow the rules of vanilla trees. In fact they're slightly more aggressive at spawning than vanilla trees. The only difference is you can't grow them with Bone Meal.
3) The world height has no effect unless it changed in 3.0.3 without me noticing, so I'm still willing to bet no.
That kinda depends of the size of the nodes, smaller nodes need to be closer to start merging, while larger nodes have a stronger pull, its kinda like they had a sort of (magical) gravitation field.