There's surely a lot that could happen. I decided to replay the vanilla SP, and I'm almost at the Tekagi part. By this point, there are supposedly Rheinlanders showing up in every system. And they seem to like blocking people in.
Thus my thought is if their advance slows, they'll just sit at the connecting points for all the left over systems and shoot anything that tries to leave. So there could be an area of humanity safe (I think the Taus would be a better bet), but there wouldn't be that much for them to do. None of the systems are self sufficient enough to survive in large numbers for too long, and at that point they nomad forces could rebuild and run them over again.
I would think smaller groups would be able to hold out for much longer (no point is stomping out a few dozen when you control all the major planets and station). I imagine they could try and go visit Sprague and Pygar and all those fun places to try and learn more about the situation and see if there was some other way to deal with the threat.
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