To recap, I think Assad needs an arc from the Turkish border, around Lebanon to the Israeli and Jordanian border, including Damascus.
This preserves Syrian claims to Turkish territory, centers the country on the Allawite homeland, keeps the UN seat by continuing to be the recognized Syria, retains the option of being hostile to both Israel and Jordan, and controls the land borders of Lebanon.
Everything else to the east is abandoned to whoever wants to govern it. Let's look at the map:
I figure Assad needs to control the provinces of Idlib, al Ladhiqiyah, Tartus, Al Qunayurah, and Daria; the city of Damascus; plus the western parts of Hamah, Hims, and Dimashq provinces, arcing from the eastern border of Idlib down to the eastern border of Daria.
A 2004 census gives those provinces a total of 10.1 million people. I'll call it even by assuming populations increased but that the eastern portions of Hamah, Hims, and Damascus will reduce the population to control--but not by much since a lot is fairly barren.
With 75% of Syria's population Sunni, in theory Assad could gather the entire 25% Allawi, Shia, Christian, and Druze in the Core Syria and increase their share of the population in Core Syria to about half the population. With the guns, money, and secret police, I have to believe that's a base of support capable of suppressing resistance to his rule in this Core Syria.
It is commonly reported that Assad has 200,000 loyal ground forces (army, secret police, and loyal militias/gangs). Standard counter-insurgency states that you need troops equaling 2% of the population to control/protect them. 200,000 is almost exactly 2%. I have to conclude that Assad would have the horses to control a Core Syria.
If those troops can keep fighting for the duration without a rotation base to relieve them.
What I also don't know is what the logistics for this move would entail. Is it possible to move supporters from outside the core? Can strategic assets be relocated (chemical weapons, ballistic missiles)? Would Assad need to ethnically cleanse areas of Sunnis to make room for incoming supporters? Would Assad want to destroy the abandoned areas as much as possible to deny easy use by whoever takes over? And how would a Core Syria's economy function? Would it become even more reliant on Iranians and possibly Russian support? Could China be enticed to provide diplomatic support and money?
Retreating to a Core Syria would be a major decision with a lot of difficulties to overcome. But it might have the advantage over trying to hold everything in that it doesn't end with the Assad family hanging from lamp posts in the Damascus public square.
The option is better from the Assad clan's perspective, no?
