Christianity as a whole has tons of Pagan traditions carried forward, so I wouldn't give the Spanish that much credit. If you want foreign people to adopt your religion you incorporate their practices into yours.

That caring how the city's name is spoken/written is just a dog whistle for people who want to be exclusive to feel special. It's not like they're trying to get the name right because they really care about the saints. They just want to feel like they belong more than others, because it makes them feel like they have a stronger identity. But it's just a word for a place; we all get what 'San Fran' means. Getting uppity about is just performative insecurity, like grammar nazis.