On the subject of 'not enough Mormons to control the US Southwest', what about having them abandon their racist doctrines and polygamy? Might that give them more converts? It would definitely help if there was no Mexican-American War and if the California Gold Rush is delayed as much as possible... even if the LDS don't take California, it'll still be beneficial to them to not have such a populous neighbor, even if you just delay it by a few years.