And the assumption is that the Iranian interests, in this case, do not align with Iraq's interests in long-term stability? A "federal region with near-independent powers" sounds like a description of Kurdistan.
It is also the recipe for a divided, militarily weak central government.

My own assumption is that Iran's maximal solution is a stable but weak Iraq which which it can dominate through its tight relationships with Shiite religious parties and the Iraqi Kurdish parties. The best way to accomplish this is through a strongly federalist constitution and a weak central government.

It sounds like the criteria of success hinges not on what is accomplished, but how. ISF was only able to get this far because Sadr stood down. Nassirya may be under control now, but not before JAM seized the initiative. Why did Sadr tell his goons to stand down? Was he feeling generous? Was JAM incurring too many losses to sustain? Was he acting upon advice from Iran?
My own feeling is that Sadr did not want to force the U.S. to intervene on the side of the ISF. He knows from 2005 that taking on U.S. forces head-on is suicide. Routing the ISF comprehensively in Basra would only bring in more airstrikes and possible U.S. intervention in Baghdad and perhaps even down south. An outright military defeat of the ISF and the US is not in the cards, but a military standoff combined with a well-orchestrated political victory that shows the hollowness of Maliki's posturing looks pretty good. Not a bad place to be with local elections on the horizon.