Just throwing stuff out here to get plotting going, but I am extremely intrigued. I dig the tone, and I like that this still leaves a lot of room for us to continue what's been going on in endgame to their logical (or extreme) conclusions.

I have a couple of questions, though, and I'm not sure if I totally brainfarted and missed it when you put it up there, or if this was a detail you wanted to work out with the rest of us (or even just hold back to bring out as a plot twist):

-Is this assuming that the Earth defeated the invading aliens?
-Whether it did or didn't, where are they after that invasion? Are they going to try again?
-Are these symbiotic-parasite-invaders going to be working secretly, like yeerks or parasytes, or are they going to be more upfront about it?