But you're totally free to interpret the ooniverse as you wish!
I'm currently trying to imagine what it would take /to/ accomplish the 'non-humans go away from G1' scenario. Looking at Galaxy 1's populations, my quick estimate is that there are roughly 300 billion non-humans in G1. Assuming an average of 75 kg apiece, that's around 25 billion tons of slave-equivalent cargo. The largest
vessel I can find holds up to 3000 tons; somewhere north of 8 million trips would be required. Which would be 40 billion credits just for the galactic drives to G2 - 220 billion if they cycle the same ships around the other galaxies back again. And the organizational issues would involve convincing a hundred distinct cultures, across techlevels, organization styles, and species, all to make the same move at the same time.
It all seems... rather less plausible than "some human supremacists wiped them all out, and managed a large-scale coverup". Which is a scenario I'd rather avoid using.
So, looking at options for what else might be going on... there's the rather significant fact that after this point, galactic hyperdrives stop working - or, at least, people from G8 stop arriving in G1, so people in G1 start assuming that using a galactic hyperdrive is an expensive form of suicide, and so stop building the things. If that's a physical effect, then that implies something along the lines of Star Trek Q reeingineering spacetime, which is a /bit/ more of a mouthful than I really want. But there's also the extreme implausibility of a galactic-scale teleport which nudges its user along a chain of exactly 8 links; sure, it's good for playability, but I'm trying to handwave a self-consistent set of rationalizations here.
One approach that seems to have promise might start with the 'Big Lie' that witchdrives are physically limited to 7 lightyear jumps, and that GalCop enforced that limit for (insert nefarious control scheme here), until it fell and fully-capable drives became available in time for the next games. If we've got a single organization already maintaining that sort of large-scale fiction, then tweaking this conspiracy to also be doing something with the intergalactic drives requires much less suspension-of-disbelief than a lot of other theories. Eg, what if there are literally no limits on such drives, so that a backyard tinker can open a wormhole large enough for a whole planet - in order to keep hostile governments from teleporting every planet they don't like into stars and/or intergalactic space, this form of conspiracy might actually involve conspirators who aren't sneaky-evil-for-the-sake-of-evil. ... Until, say, collapsing economies lead to internal pressures among conspiratorial subgroups leading to differences of opinion, leading to a conflict circa 3165, in which one group tries to wormhole away all G1's planets, another group brings all the human planets back to G1 (and replacement planets for all the nonhuman-inhabited ones), and a new stable set of standoffs are negotiated between the formerly-united conspirators, part of whose terms are for G1 to be left alone.
... Okay, so it's shaky, missing a lot of detail, and needs a lot of improvement. The two virtues it has are that it simplifies the physics a bit (general universal laws - eg, "galactic" hyperdrives can go anywhere - are preferred by Occam over weird cases - eg, galdrives located in G1 can go to G2, etc), and it doesn't require any of the people involved to work against their own perceivable self-interest.
Anyone have any better ideas?