Indeed, invading the American territory would be very hard, specially if Mexico is losing the war, being occupied. But the guerrilla warfare could tire the Americans enough to leave most of the Mexican territories.
A usually non mentioned problem of the "All Mexico movement" (that defeded the full anexation of the country) it's that Mexico had a large enough population to resist assimilation (unless genocided) and even it could backfire with A Mexicanization of the US.
In an 'all Mexico' scenario I think that it's very likely that guerrilla movements would arrise, more so if we consider the strong anti slavery sentiment in Mexico.
Perhaps America was really lucky with the Gauadalupe-Hidalgo treaty, avoiding a hard occupation and a very important shift in the etnical and politcal composition of theit nation.
A complete annexation of Mexico would not "Mexicanize" the US. Without genocide, it'd result in a US-US and a Mexico-US. Remember that Austria wasn't destroyed demographically by its empire, there's no reason to believe that the US, who has more people than Mexico at this time, would be assimilated. Influenced, sure, but Mexican culture isn't going to dominate the US or anything.