You forget about the cartels, deeply entrenched with the corrupt Mexican government. In months anything along the border, on either side, will be used to launder money and the whole thing will fall apart.
Either that, or the cartels sabotage everything in order to maintain power, or possibly just take everything over with violence. The Mexican armed forces are worthless there when it comes to dealing with cartels. Do not count on them for protection, and if our forces take out cartel henchmen, the whole thing still falls apart, because the (again, entrenched within) Mexican government backs out due to perceiving it as a national security threat. With us being the threat.
Nah. Not gonna happen.
Simply have the US own defense of the entire zone as it is our own economic and security interest to do so. We already do this all over the world. Our own southern border should be a piece of cake.