Well, irregardless of how it happened, the American defeat would be a rebuttal of the Manifest Destiny; And maybe they'll try again ten years later. An American Civil War will surely be hastened due to the fact the war goals were never achieved, and the slavers will attempt to expand it into the remaining territories.
Mexico, if it managed to unite against the invaders, will disunite again on petty differences until the equivalent of ATL's Reforma rolls in. I think the Conservadores might win this time around, but Mexico would be severely weakened.
Another possibility would be if the U.S. annexes all of Mexico. Yes, Mexico lost the war, but it will surely win the peace in the long run, with the insane amount of Mexican and Indigenous uprisings the US will have on its hands. Remember that Mexico's modern northwestern states were inhabited by a lot of indigenous warlike tribes that didn't liked at all Porfirio Díaz when he seized their lands. Though, in this case, both Mexico and the U.S. will screw themselves so badly that both countries will be shattered to hell and back.
Anyway, I've been thinking on making a Collaborative TL of this.