I was trying to work out the backstory to module 1--how was the destruction of the first colony arranged?--because my player is really into detective work and I can't handle that unless I know what actually happened.
This led me to consider the scene that the module glosses over as "Ochymua offers the colonists to Onthooth in return for information." I think it went like this:
Ochymua: I'll offer you a bunch of humans in return for some information about local affairs.
Onthooth: Sounds great! How will you deliver them?
Ochymua: Just send your faceless stalkers to their town and impersonate their leaders, then have your ulat-kini pick them up--you can steal their boat--and take them to your base.
Onthooth: In other words, your entire contribution to this is telling me to do it.
Ochymua: Not at all! I have captured two of their leaders. I'll graciously let your invisible stalkers have a look at them, to get you started.
Onthooth: Okay, you have a deal.
I think this is actually plausible, because regular aboleth don't really want to mess with veiled masters. But wow, what a bargain.... Ochymua gets what it wants and Onthooth takes all the risks and does all the work.
Ochymua likely offered information in return. Most importantly he can read minds, giving the Faceless Stalkers very important information they probably wouldn't get otherwise. I don't have the book at hand to check this, but possibly the Aboleth also simply didn't know about the settlement. Giving the information there are suitable test subjects nearby alone might have been a favor.
And of course the fact that Ochymua could simply read the aboleths mind to get all the information needed. Offering collaboration was merely a courtesy.
Yeah, Ochymua has all the cards in that particular confrontation. (Though I doubt it had much information to offer, having been out of the loop for 10K years!) Still, the image of this conversation makes me laugh.
In my game it ended up that those are Ochymua's faceless stalkers, not Onthooth's, which helps. Ochymua knew where they were living and went there to recruit them. So it actually did add some value to the colony-capture project.
I am playing all the other bad guys as scared shitless of Ochymua, which has percolated down to the PCs (who love to talk to baddies, and had a surprisingly good conversation with a faceless stalker--it's a long story). I don't think the stalkers know what Ochymua is, but Onthooth does, and is not at all happy. (In general I doubt regular aboleth like veiled masters at all, but there's not much it can do about this as it is severely outclassed.)
It's also clarified in later books that most of the first group came over while under Ochymua's Domination. One of the NPCs from book 3 talks about it. So Ochymua brought a bunch over and then let the faceless stalkers replace them and take over everything else.