So given the obvious barriers against international partition and occupation (not least when you want Turkey and Saudi Arabia to do the legwork, and cooperate while doing so), why not aspire to that specifically?
What you presented seems to be the problematic occupation and partition,
plus encouragement of the rebels and Assad to grab as much land as they can from each other that doesn't constitute a DMZ?
Since if the (unified for our scenario) rebels and Assad are pushed to concentrate on fronts against IS, leading to loss of territories by IS, then why wouldn't Assad and the rebels subsequently contest the ground that IS has abandoned? You would essentially need to continually expand the DMZ, and maintain considerable military and security deployments
for the sole purpose of preventing DMZ violations. 
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