Is Chiapa de Corzo outside of Tuxtla, up in the mountains near Sumidero Canyon? That might be far superior to overcrowded Guadalajara, DF, or the intense heat of Tuxtla.
I forget Playa del Carmen - is that near Ciudad de Carmen in Campeche, or closer to Cozumel and Cancun? If it's near Ciudad, is that the flamingo preserve, and the gulf water is hot and calm and shallow?