OK, let's start with the gas tank.
By a huge percentage, most of our oil comes from Mexico and Canada. Hardly any of it comes from Iraq.
There's already havoc in the area, and every report says the longer we stay there the worse it gets. So, being there isn't a solution to preventing havoc.
You are probably correct in than the absence of US forces will eventually lead to a situation where other neighboring countries get involved. But that's going to happen whether we stay there a year, or 5 years or 50 years. How long do we want to be an occupying force? Our involvement in Iraq has already surpassed our involvement in WW2 as far as duration.
We have bases in Japan (Okinawa) and Germany, but those counties are stable without our continued assistance.
But that's a good question. How long after the end of WW2 did it take Germany to stabilize? What was our level of involvement? How much did it cost? Those are reasonable estimates to start with.....
But remember that Germany was not historically caught up in a civil war that had preceded the Nazi regime, the way Iraq's sectarian violence preceded Saddam. So, the time required for Iraq to become stable, even without any interference from other counties, is likely to be much longer than Germany.