Didnt Mexico during the Mexican Revolution separate their church as the "Mexican Catholic national Church" for a few years? This is more similar to orthodox declaring their church autocephalous and was really shortlived but it happened.Does it have to be an established protestant branch? Because you might be able to get some splintering when Papal Infallibility was declared, perhaps the authorities in Spain or France decide to seize the opportunity to establish a "Latin Catholic Church" in something like the Anglican Tradition. From there, just use the POD to keep the Spanish Empire together, maybe even in Union with Portugal, and then wait for the Papacy to do its thing. A lot of the ground work had been established earlier, so it seems like the official declaration would happen eventually. Decent chance of it happening anyway
Does it have to be an established protestant branch? Because you might be able to get some splintering when Papal Infallibility was declared, perhaps the authorities in Spain or France decide to seize the opportunity to establish a "Latin Catholic Church" in something like the Anglican Tradition. From there, just use the POD to keep the Spanish Empire together, maybe even in Union with Portugal, and then wait for the Papacy to do its thing. A lot of the ground work had been established earlier, so it seems like the official declaration would happen eventually. Decent chance of it happening anyway
A few incompetent and/or scandalous popes could cause conversions to protestantism. If they exclude some majority-catholic ethnicities, conversions from them would probably increase.
What do you mean "exclude majority ethnicities"? They'll take the tithes from and give communion to anyone, even if they ignore any of these ethnic groups political-national demands.
It is not in Europe, but a US annexes all-Mexico scenario sees Protestantism make inroads among Mexicans? There's more preachers around, mixed marriages, some anti-Catholic sentiment anyway in some circles and it can be an aspect of social climbing for some.