I agree with your first point. The nebula in essence is the atmosphere required for the barnacles to live in. The barnacles seem to be more like a type of harvesting point for these meta alloys.
The UAs are different though. The UAs dont look like seeds at all but rather probes looking for something. If anything looks like a seed, it is the meta-allow thingy that people were taking off the barnacles. That to me, looks like a seed for another barnacle.
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My best theory right now imo, and my best lead is that the UAs found something in Merope which we have yet to find. I suspect it is either a really rare USS or a celestial body that can not be discovered with scanners. I suspect that most people there spend most of their time looking on the planets and not looking for USSs as much time, or a celestial body that is not detectable with the lazy scan all approach.