How did the Archon gain control of...

Nope. You're saying that someone who never been in  kett base nor seen what the kett forces are in other places and the fact states that every Argarain that tried to do so never comes back is going to know all the detail of what the kett have and don't have in general? And you complain about holes in the plot?

I did not know about that after completing the game since I haven't completed any missions after the main story. I figured they would regroup before continuing. With Primus taking over, I would believe he would call his leaders for reinforcements to replace the ones that were lost. I believe the leaders would send a larger force to the cluster to help Primus.

Anyways. The kett are the biggest threat even after Archie dies. They know the locations of all outposts. They know the location of the Nexus and Meridian. Having a larger force on hand, will make them extremely hard to stop. How long would it take to send more forces? Don't know. I'm curious what will happen with them in a dlc or the next game

You're attempting to place the Archon into s neat little box defined by "most kett", or "kett generally do X". The game shows quite clearly that the Archon does not adhere to typical kett norms. He's been pursuing Remnant technology for decades, which is something that is so aberrant that it's causing rifts within his forces. The question of how he understands SAM's tech is a good one; but lack of an in-game answer doesn't mean there isn't a good one. Would it have been "good writing" to feature a scene in which the Archon explains his understanding of AI? Sounds superfluous to me.

My point is the archon is the aberration. He alone tries to understand the remnant. He only has his knowledge/experience to draw from. The game doesn't have to explain his mastery of AI in-game: There exists a codex. It would have been "smart writing" for BW to support their ludicrous plot point, now plot hole in my mind by including the archon's mastery of SAM in the codex. It took Alec years to build and perfect SAM, but Alec must have been a retard bc it only took, what, a few weeks for the archon to learn and control everything--and all from Ryder's memory flashes of family fun time? I guess the kett really are superior, and masters of the universe.

There are other examples that show BW isn't exactly on top of things. People point out how the science of ME often breaks the laws of physics. I work the the medical field, and wondered what the hell kind of physical exams Lexi was doing, and questioned the ethics of her giving Ryder a surprise shot. Sure, she's an alien, but the Initiative is human founded, and she seems to subscribe HIPPA laws, mostly, but hey, whatever is plot convenient, I suppose?

And how is that relevant what the Kett usually do, when we talk specifically about the Archon? The Archon will use any means neccesary to obtain Meridian for DECADES now. 
That's why you can see Kett FIGHTING other Kett.
That is why you can make a deal with the Cardinal, regarding the Archon.
That is why the Archon simply didn't blow up the AI on Voeld. No he has basically built Fort Knox above it to do more research.

It's like saying BW doesn't care about the Codex, because Alec Ryder build SAM. It's pretty much established that AI's are illigel in the Milky Way. You humans are all racist!