I had what feels like several radical thoughts. The UAs were originally found in convoys, right? And the wreckage around the Barnacle in Merope is an Anaconda with Caine-Massey crates surrounding it?
So what if they aren't alien? (Sorry if this has already been hypothesized.) What if they're just genetically created organisms that had some... unforeseen mutations? There's quite a few things we don't know, including who is transporting the UAs and why, who are Caine-Messey and what are they doing shipping around Merope (and for how long)?
Also, where did the name "Barnacle" come from? From what I can gather it came from the description of Meta-Alloys that cites "interstellar explorers", which means that in itself is a hint. Following that logic, barnacles on Earth are the non-motile stage of an organism that before that can move around. During their larval stage they swim (or drift) finding somewhere to anchor themselves (and interestingly, they do it with their antennae). So, what if the UAs are the larval stage of the Barnacle, looking for new homes, thus, they aren't point *towards* Merope, they're pointing *away* from Merope, the Shell is the distance they've traveled already (notice in the compiled image showing the shell the locations found in 1.3 are closer to Merope than 1.4) Such a life-cycle would support their propensity for low-g, airless planets (they can "eject" the larva into space with minimum energy), and speculating on why nebulae: because free-floating bountiful particles. They're like the algal blooms of the oceans (coincidentally theorized where life began).
Also, in case this hasn't been brought up (surely it must have been), when particles collide they often release visible light-waves: its quite possible the UAs are releasing particles that interact with the stellar medium in a way that produces the "fireflies", and explain why a ship AI would register it as Toxic cargo: they're radioactive, expelling an energy source that our electronics are not/cannot be shielded against... which is why the meta-alloys saved Obsidian Orbital.