Worry about your fiance. Latan American girls want to land an American guy and go to great lengths to do so. I served in Peru and this doesn't sound right. Three years after coming home and he continues to write them...sounds fishy especially with the con amor sign off. I stopped writing the girls I knew there because it was clear what they wanted from me. He likes the attention but seems to lack discretion to know to cut this off.
You're heading for trouble, deep trouble in your marriage. Faithful Mormons want nothing more than to convert their non-LDS spouse (maybe not now, but that desire will kick in) so you can be his eternal companion to make spiritual babies and him become a God. He'll feel empty at church and for him, will have settled for much less than an LDS spouse would offer in the eternities. As a faithful Mormon for many years, I knew of no LDS/Non LDS couple who were happy. All ended up wanting only to change their spouse.
I've since left the church and often run into women who want me to give them the magic formula for leading their deceived husband out of Mormonism. Mormons are spirtually blind and deeply so. The more non-LDS spouses learn about Mormonism, the more they loose respect for them. Women I know who are in this situation just shake their head at the stupidity of becoming Gods, LDS salvation by perfecting themselves or getting special ordinances, follow the prophet, time spent in callings, the judgmental culture, etc.... The more you nag, the more entrenched he'll become in his beliefs. Or if he leaves, his family will view you as the reason he lost all the blessings of the "restored gospel." You'll have become a little Satan in their eyes.
It's heartache on both sides and I doubt your marriage will survive this. He might feel tolerate now, but that normally changes. Give this some very serious thought.