I've wondered a few times, but here's the question: Planets like Ecumenopolis and Vorigar are kinds that can only be found by using the mission arti.. Other than it's location being hacked from your database, is it possible for them to be scanned by other players at all?

Yes. When you do the mission, a planet is created. And shows up on your scans. However, you can't scan one that hasn't been scanned before.

Also saying yes, and from experience. I have picked up a few extra aphotics from scanning and invading.

I actually own an invaded Aphotic. Vorigar X1255.

I, at one time, had a 4th Aphotic and still have a couple extra Gaias and several Crystals (having not even completed Pawlacite Thaw).

Yeah, planets like Gaias or Irradiated are simple to tell that they're scannable.. They're just upgraded planets you can scan on a daily basis. that don't get added from scan data artifacts. Thanks for the pointer, deffo still plan on keeping boosted def/atk on those. All I need now is to manage scanning myself a natural dyson.. Too bad I already have all my rifts lol