Is the FSA even demanding partition? I never once heard them ask for it.On the contrary, Assad keeps insisting on it, to the dismay of the Russians who want some sort of peace process.
http://www.nytimes.com/2016/06/08/w...iant Assad Vows to Retake Every Inch of Syria
Also, the other part of your post doesn't make sense. The Iranians would have a clear run to the Mediterranean and Hezbollah if the regime again controlled all the territory. At the moment it's much more difficult for them. Why on earth would they not want Asssad to retake the whole country?
What other State would cede territory to Islamists bent on sectarian supremacy?