How rapid was the decline of indigenous languages in Mexico and Central America? Was the decline due more in part to the era of Spanish colonization or the post colonial formation of “Mestizaje” as a form of national identity after independence?

You may find this earlier fine answer by /u/dhmontgomery on this topic interesting. It details how Spanish became the dominant language in Mexico beginning only with colonial reforms in the late 18th c. and especially following independence.

I also added some points related to your question in this recent thread, e.g. on Nahuatl becoming a lingua franca under the Spanish. Hope this helps!