How can Syria really contain the area where they'd use this stuff? They're far from 20th century knowledge, much less 21st century knowledge.
It's not like radiation, it's 'persistent' in chemical weapons terms but Sarin is delivered in an aerosol form because it is essentially a slightly viscous heavy liquid, 'Aerosol' means droplets and it falls out of atmospheric suspension and deposits on surfaces within a couple of hours after the bursting charge creates the aerosol cloud (Depending on winds and the amount used). The droplets that are deposited remain lethal for a pretty good while, up to weeks, but UV light and weather break it down and disperse it like other complex organic toxic chemicals. Fictional descriptions tend to grossly exaggerate its effectiveness because 'nerve gas' makes a really great
Deus ex machina to wind up all the loose ends in a story.
As far as Israel goes, those points as a reason to attack now are illogical because Assad could have mixed it and dropped it on them any time in the past ten years or any time in the future, whether he does or doesn't dump any of it on the rebels (Who are as big a bunch of scumbags as the Ba'athists are) now. They're independent decisions, not that he might not want to attack Israel. Also, I don't see 'Attacking anyone who might attack Israel' as a sane foreign policy strategy for us.
Large-scale intervention in Syria would turn into a cluster**** that would make OIF/OEF (<- which is STILL an on-going cluster**** itself) look like Napoleon at Austerlitz by comparison.