I can't help you with the social system, however the first Covenant-Human contact was with a one maned human freighter, then with the Human colony world Harvest, in 2525 IIRC. The Covenant ship was using a device that was able to detect Forerunner relics. However, instead of finding relics, the device found the humans, who were the rightful Reclaimers of all forerunner technology. The Covenant didn't realise that the machine wasn't telling them to "Reclaim", it was pointing them to the "Reclaimers". This information was kept a secret by the Prophets of Truth, Regret and Mercy, due to the fact that it totally undermined their whole religion, and would lead to the downfall of the Covenant. My info is a bit sketchy on this one but I'm pretty sure the Covenant was formed following a lengthy war between the Sangheili (the Elites) and the San 'Shyuum(the Prophets). The Elites ultimately lost the war but the Prophets saw potential in them and allied with the Elites to strengthen their power. This is why up to around halfway through Halo 2, the Elites were the Prophets Honour Guards.