I'm assuming that the Mormon interest in Mesoamerica will still happen in the 20th C. in this timeline (there are strong structural reasons for it), which means that ultimately the Mormons will probably interpret Mesoamerica as the promised land and view their eventual remove their as providential. Mormon beliefs actually makes them unusually fit for being an alien, anglo element in Mexico that nonetheless is strongly patriotic and committed to the Mexican country.
Mormons moving to the Yucatan and getting involved in the Caste War would be fascinating as heck. Mormons in Mexico in general would be amusing. Imagine el Presidente Romney. Someone get MormonMobster to do a timeline, now!