So, some wild guessing...
...and it's likely stuff that you've all noticed before, but anyway.
So Petra Prawatt in Ancestor is obviously the origin of the Prawatt (I noticed her in the first read-through of Ancestor, but didn't put 2+2 together, but the purple hair made her stick in my mind).
The All-Pro describes both the origin and near-genocide of the Prawatt on Earth taking place around 2015/2016.
The last chapter of Contagious talks about a five-year time skip (a line something like "over the next five years"), so that puts Pandemic right in that time frame.
I can't remember if there were any Triangles left outside of the immediate Detroit kill zone, but if there were, the wilder part of the guess is that the gate operates in both directions, and that's how the Prawatt escape Earth.
However, I do think there has to be some alien intervention to assist them in getting off Earth - it's not we're launching things into space every day, and even if they were to hijack a satellite or such, they'd be limited in how far and how fast they could move (even if they were able to improve on Voyager's schedule leaving the solar system, it's still nothing compared to punch drive).