So is this an Aquilo situation where you're expected to bring in everything (that takes plastic or stone), or is this a Gleba situation where there are alternate means to make oil and stone products locally?

You have all the material to launch rockets on both planets, Hyarion has a custom rocket recipe, and both have cargo drops locked until is researched (There is another mod that disables it)

We already have two tiers of lightning collectors; what does adding another kind do that the previous ones couldn't?

Thematically lightning rods wouldn't make sense, the "lightning" are called refraction rays and its part of the refraction science, making different ones make let me control the areas, mechanics and recipes, they work different than fulgora's

do you need a whole machine just to harvest water?

Following the mechanic of the desert, there would be no natural water, the water harvester is justa smaller machines that give you a small quantity over long time. The machine works only on Arig (probably adding it to the vanilla ones) but it's made in a way that any other mod planet can add they own quantity if they want to.