Sarin is not that difficult to deliver. The relatively low number of fatalities speaks to a inefficient system.
Only if you believe that the delivery system was actually "sophisticated" missiles.
There are two problems with that assumption, but I'll let the proponents chew on it. Why should someone else do all the thinking?
Exactly what makes the delivery of Sarin gas too difficult for rebel fighters?
If you dont think the FSA or rebel fighters dont have many of the same weapons as Assad troops, you arent paying attention. DIY weapons and delivery systems are being used by both sides in this conflict. Rebels have captured whole towns, including stock piles.
Here’s some interesting analysis.