A source told me that the Arab world often uses middlemen to coordinate outcomes. I suppose since you can't trust others (rightly so said Hobbes) then the use of a middleman can come in handy.
Maliki need not and should not go right now. No leader should step down under such conditions.
What is needed is the right diplomacy. Example: the US or the UN should use an ambassador to Iraq or a special one to shuttle between the various contenders. With some initial confidence building measures at first and then leading to something like an Office of the Purveyor to continue the Middleman role.
More likely is a de facto partition of Iraq with ongoing low level violence.
Sending in US spotters or spies will piss off someone. If they spy on Iran from Baghdad to see what it is doing in Syria that will piss off Baghdad too. If they spy on Sunniland to bomb targets that will piss off the Gulf States.
You see the goal of the US is instability: don't let anyone win, not Maliki (would help Iran), not Assad (helps Russia), not ISIS (helps Saudi and ould scare Israel).
But the backers involved want these factions to WIN. Unlike the USA, they dont want the pot to just keep on simmering forever.
Keep in mind too the track record of the US in that region has been a general failure. Things always seem to backfire over there.