Technically, their knees aren't backwards, they just (in terms of human anatomy) stand on their toes and have abnormally high ankles.
As for the topic, I'm not quite sure, but I'm betting all of one race is gonna get pwned. The covenant don't take prisoners, let alone make friends with them. And how can they reconstruct Earth? Only the forerunner are capable of such a thing. And what make you sure Earth is going to get destroyed in the first place?