I also agree that they would have to solve the problems amongst themselves (ie. 'pressure cooker'). There is a strong Nationalist bent amongst all Mexicans and, like Iraq, any US involvement would galvanize many to fight the gringos no matter how well intentioned we are in getting involved in their affairs.
However, the problem with your scenario is that things COULD get worse with a real Socialist, ala Fidel, getting into power down there (IMHO). Any unstable or outwardly belligerent neighbor would not be to our advantage. Nonetheless, maybe the situation could be 'steered'? The problem here though is who would do the 'steering'. (Another CIA-engineered coup to stoke the conspiracy-buffs?)
In the end, I have no good answer to this problem.
Which makes our own government's stalling to do something about the greatest security threat to this nation even more a crime. What exactly is the down side to building a defensive fence on the southern border? We are spending literally hundreds of billions of dollars and are willing to risk thousands of American lives in Iraq under the name of "security", yet if a Fidel Castro or a Hugo Chavez nabs political power in already corrupt and impoverished Mexico and directs the frustrations of the Mexican people against the US even more that it is being directed now, without the security fence, we are left to our 2nd Amendment.