^ I'm assuming you know about all of these areas?
p.s., could you dig up a map of where these churches that "jumped ship" came from? What areas? Or what demographics?
This might not matter as much as it seems. The majority of Guatemalans are ethnically mixed (what others would probably call mestizo, though there they're called "Ladino", not to be confused with the Judaeo-Spanish language commonly called that), regardless of what language they speak (though the majority of Ladinos speak Spanish as a mother tongue, just like in Mexico where most people are mestizo but only speak Spanish). I think that map might be a little bit deceiving because you'd think from the size of the territory, for instance, that Mam people would be more than about 8% of the population (about 650,000 if I remember correctly; I studied Mam language as an undergraduate...it's pretty wild; gave me a big headache). Q'eqchi', believe it or not, has even fewer speakers in Guatemala proper (400K, but total in all countries is several thousand more), despite being spread out over a much larger area, if you believe the map.
So chances are that the people who jumped ship are probably Spanish-speakers and very likely Ladino identifying, no matter where they are. And, if my informal survey of the three little old Guatemalan ladies I used to live next to in California is anything to go by, even people who are very obviously less European than Indigenous are unlikely to have 'asserting indigenous cultural identity/bucking colonialism' high on the list of reasons for leaving one church for another, though I don't doubt that this does happen. Many years ago, I had a class with a professor who had worked in Guatemala in the 1970s, doing language documentation work among one of the highland Mayan peoples (I don't remember which one), and he said that the people he talked to had great respect for the Mormon missionaries that would frequently visit their villages. My professor was confused until one of the Mayans told him "the missionaries are the only ones who actually learn our language and can speak it. Everybody else talks to us in Spanish, and we don't like that."
A more colonially-minded/"white" (ah, heck, why all the euphemisms!) religion than Mormonism, I can't imagine...what with all the "Adam and Eve lived in Missouri", "upstate New York is holy ground", and other odd ideas in it...it's very far from being an ideal vehicle for asserting or preserving native Mayan culture, and yet here was a place and time when Mayans found it very attractive, because the missionaries who were sent there met them on their own terms. Something to think about, perhaps, in our rush to believe the best of these Orthodox conversions stories.