Just a few thoughts about the reapers swift arrival-
Many are asking "If they could have got back so fast by normal means, why try to take control of the citadel or use the collectors?" There are a few theories out there that a Galaxy has a sort of 'barrier' that would make travel into and out of a Galaxy very difficult. Perhaps this 'barrier', maybe a Gravitic distortion from the sheer mass of the Galaxy, makes it nigh on impossible and potentially destructive for the Reapers to try FTL into the Galaxy. Also, the Citadel is the heart of the Mass Relay network. They wanted that so they could paralyze the Galactic community, who have not even tried to build their own Mass Relays at this point.. Flying into the Galaxy in this way is likely a last resort measure, and attacking humanity makes good tactical sense. Their 'champion' has defeated their plans twice, they have grown in power in the Galaxy remarkably fast, and their expansion, while not on a par with pre-genophage krogan, is still impressive, therefore why they have needed to colonise so much of the Terminus Systems. also, one of the Codex entries makes a note that, compared to the other races, humanity has a very low recruitment percentage into the armed forces (3%, I think). Therefore, as we see the human armed forces are a force to be reckoned with even with that, the human race has a lot of potential to be a threat, if allowed to mobilise.