With the Iraqi congressional system passing the vote to show the door to the American military.....my humble guess is that it'll be done rather quickly (maybe in less than two weeks).
The collapse of the Iraqi government? Well, I would anticipate that various pro-Iran groups will come calling and a complete shutdown of the current government will happen within three months of the Americans (and everyone else) being gone. Mass chaos, civil conflict, pro-Iran versus anti-Iran religious groups.
The potential split of Iraq? It goes to three key groups: Shiite (the southern-eastern zone...maybe 25-percent of the nation), Kurdish (the northeastern 10-percent of the country bordering Turkey and Iran), and the northwestern 20-percent belonging to Sunni Muslims. The remaining 40-percent? Mostly desert, bordering Saudi Arabia with oil wells. None of the three groups really play into this region.
It is conceivable that Saudi Arabia would care about this one-third southern district bordering them, and would try to create a military force to cordon off the area.
Oil access? The chief tanker point is down in the south, next to Kuwait. That operation would rapidly shut down.
Both Kuwait and Saudi Arabia having worries? Well, here's the thing....Iran and Iraq have never really recovered from the war period of the 1970s. The US might have provided a significant number of SUVs and APCs, but there is no offensive capability in Iraq, and if you divided things up among three groups....it makes them more or less.....a zero threat player.
Iran? They'd take the situation and quickly pump up the Shiite folks (their friends) and try to cause unstable conditions for the Kurds (who used to like the Americans until we left Syria), and the Sunnis (pro-Saudi Arabia).
By mid-2020.....I would suggest that Iraq is mostly finished off and three different nations will come to exist. For the Kurds....it'll provide this doorway for a nation to finally exist. Sunni-land being a US protected area? Maybe.
All of this leading back to the silly Iran-Iraq War of 1980, the lack of resources for Saddam's crew after the war, the threat of invasion to get the neighbors in the Middle East to rebuild Saddam's Army? Yep....all leading back to that era. Toss in the idea of conquering Kuwait as a stupid move, the US-Iraq War, and the eventual second war (where Saddam was taken down).
If Saddam were around today? Virtually everyone connected to the pro-Iran lobby group would be put on notice and if they failed to grasp the threat....would disappear overnight.
The odds of some targets being struck by the US as Iran attempts one or two stupid 'pay-back' situations? It's more than likely and Iran will go and beg off of Russia to get the latest in defense toys....with what little cash that they have left.