Pairing is weirdly inconsistent; I've had a multiple planetside pair to the same platform link upon arrival, as well as had them refuse to pair despite the platform still having 11/12 slots unpaired. It does seem to like pairing the first new planetside construct after arrival, however.

You can "force" the pairing to fix itself if you build the planetside one first, then do a deconstruct/reconstruct dance with the ones on the platform, so this mod still does exactly what I wanted (outpost power that doesn't need to run big poles, so I don't have to defend big poles), it's just kinda clunky.

I've not looked at the code to give more targetted suggestions as to the problem, since I found a workaround already. Sorry.

If your intention is to "bounce" power from one location to another on the same planet using the orbital platform, the mod hasn't been designed for that use case (interlinks are essentially accumulators that sync their power rapidly by script), and it'll consequently be kinda buggy if it works at all. If you find a way to make it more functional in that case I'd be happy to look at it, but I haven't found one myself.

Nah the ship has a 9.6 GW reactor (on top of a chunk of solar), it's supposed to power all the disparate mining outposts remotely so I don't have to run a line from my main base which could get eaten by natives (or tiberium, in this case).

To maybe better describe what's happening, I have 12 of these things on the ship, and had only 1 on the planet; when I placed a second on the planet for a new outpost, it didn't pair until I went to the ship and did a deconstruct/reconstruct dance on one of the ship's.
The issue specifically appears to be with the pairing trigger- once it pairs it works exactly as I hoped. But building on the planet doesn't necessarily succeed in looking for an idle one in space.
Might be related to how you're supporting arrival/departure pairing, which simply makes it so idle links in space aren't reliable like idle links on the planet are. Fixing it naively might even break arrival pairing to idle planet receivers correctly.

That use case definitely should be possible, and I've replicated the strange flakiness locally. Why it's happening I'm not quite sure of; the same procedure is called for new construction and new arrival, and the necessary data definitely exists. I've uploaded the source to make it easier to flip through.