1.) That's a really hypothetical question. I can't really say anything worthwhile on the matter. 2.) I don't understand. 3.) Because Truth, Regret, and Mercy discovered that humanity was the true heirs to the Forerunner legacy. Knowing this information would destabilize and fracture the Covenant, the three of them ordered the extermination of humanity, claiming we desecrate Forerunner artifacts. They might have tried a different approach, had they known we were spread across over 800 worlds.