Mexico's winning of the war depends entirely upon if their allies can win their own theaters, and how much territory they hold at settlement. However, given Mexico is stronger ITTL, and the DSA is smaller than the USA, I'd say the DSA starts out a favorite, but Mexico is no slouch. I'd expect either a stalemate along most of the fronts (after the surprise of the initial assault) or else the DSA makes some headway, but doesn't march on to Mexico City. Either way, I don't think they'll take too much more from TTL's Mexico, as I really don't think they want to be troubled with a restive Mexican population. I think they'd be more likely to ask for Nicaragua for the canal than OTL's South Texas.