This is a posthumous novel(?) from the 70 or 80s(?) where a modern day archaeologist who is transported back to the Aztec empire on the brink of the conquest, but then changes the course of history à la Lest Darkness Fall.
La Malinche is a main character.
Not Pastwatch which involves the Maya, with the time-traveler's help, modernizing and forming a coalition AGAINST the Aztec. This book features the death of Motecuhzoma Xocoyotzin at the hands of his own people for his feeble response to the Spaniards aggression. Possibly la Malinche instead of becoming Cortez' mistress is won over to side with the time-traveler. Again, I have a vague recollection of this being a posthumously published work, perhaps completed by someone else. I may be thinking of another book.