i think the tipping points are going to be what does Hizbullah do at the point of invasion. The use it or lose it mentality may kick in and TelAviv might get hit provoking an even stronger IDF response. The other point of no return, particularly with regards to syria, is whether the IDF ventures into the Bekka which Assad would have a tough time dealing with politically if he doesn't put forward at least some type of token reponse, which might not even be necessary b/c the IDF may take proactive measure to protect their flank like i mentioned in my earlier post
It would be more difficult politically to justify an attack on Syria than it would on Iran.