Ok, I'll play devil's advocate. Let's assume evolution is true. Just how likely are we the only life form to evolve? And if we aren't the only one's to evolve, is it possible that there are other life forms perhaps much older who have managed a more advanced knowledge of physics than we have and who are capable of interstellar flight? If I accept evolution, that scenario is certainly plausible.
If I reject evolution, but embrace the explanation offered from God in the Bible, then your scenario is a certainty. The hideous extraterrestrial beings are the fallen angels who are indeed bent on corrupting man and controlling the Earth. The advanced guard are humans who knowingly or unknowlingly promote the fallen angels evolution agenda, much like the anti-war protestors were so certain the facts were in favor of Saddam.
Yeah, I believe you are an advanced guard. But not the purple seven fingered kind. More like the kind thats a pawn in other people's game and eventually gets squished under the bulldozer.
Knowlegable people accept evolution because that's what the evidence indicates. (Not to you, but you're not knowlegable.) So there's solid reasoning for acceptance of evolution. The rest -- alien creators of species who travel FTL -- is pie in the sky. You can't expect to have both ideas treated as equally probable. You're not very good at this stuff.