I thought those books were pretty hot.
Although, they seemed to be... okay, I'm new here, what's the word for when a timeline pushes one political entity over another? Anyway, they seem to be doing that for the U.S. Also for Earth, too, but since there's no contact with the Race, unlikely there's going to be contact with any other planets either, so you don't have much else to focus on.
It seems amazing that there wouldn't be any significant space travel by the end of the twentieth century, but I guess, without Race technology to aid them...
As for the temporal rerouting, what if some disaster happens to the Conquest Fleet en route, preventing it from arriving near Earth. Years later, when the Colonization Fleet arrives, they find Earth unconquered, with no sign of Atvar anywhere. Lacking the weapons capability to carry out the job themselves, they are forced to go back. If Earth doesn't have advanced space technology at the time, they might not even notice the fleet arriving, or figure it for some minor asteroid.