Stanton Friedman would shred all of your assumptions in a flash. You might check out his website.
BTW, they have been visiting earth for thousands of years.
The government evidently had some level of either awareness or contact BEFORE WWII.
Oh, right! They built the pyramids in Egypt. I forgot. And of course, how else to explain all those crop circles, not to mention all the misfits and losers who claim to have been abducted and subjected to all sorts of prodding and probing by aliens. I know, I know, how can I possibly deny the veritable MOUNTAINS of evidence? Eyewitness accounts, oddly radioactive ground/plants, etc.
Stanton Friedman's arguments are not evidence. They're not even particulary complex or interesting. But they do help him sell books and lecture appearances, evidently.