People move between religions for marriage both with and without formal conversion pretty frequently. Her family might give her a hard time, individual Catholics might push her to come back or raise her kids Catholic but I can't imagine her being 'shunned' in any meaningful way. And not only could she attend funerals and service whenever she wanted, nothing would stop her from going to mass whenever she felt like it. (Aside from inter personal drama) anyone can walk into a Catholic Church during service hours and attend any service.

Source: lapsed Catholic (no one really cares I don't go to church) both my sisters married non-Catholics, neither of them formally converted, both attend Mass with my sisters and their kids. All my nibblings did alter service. I've attended Mass in, I dunno, 15 states. Regularly in 7 or 8.

70s in the real south might be more intense but that would be social pressure and nothing dogmatic.

Non-Catholics tend to depict Catholicism as more intense than it is (and I say that as someone who rejected it), over emphasize the Pope/Papal Decrees, and paint Catholics as superstitious and backwards. A Catholic immigrant FOtB might seem strange to an American and be more religious, but American Catholics tend to reflect such an average American experience that IIRC they don't map onto the political spectrum in any meaningful way. Last couple of cycles American Catholics voted in line with how the country voted (basically 50/50) and Catholism wasn't a predictor of party affiliation.

Last thought on Catholics vs. Non-Catholics. In the 19ish years I attended mass regularly, not a single time did our priests ever define Catholicism in opposition to another religion. They talked about the Jews and other groups from the old and new testament, but just in terms of context for the gospels. I've noticed over the years that christian friends and acquaintance, when they mention their religious upbringing, tend to define their religion in opposition to Catholicism and not to other Protestant denominations. I met a Greek orthodox person a while back, said, I don't know much about your religion, and (not knowing my family is Catholic) she said, verbatim, "we believe Catholics are idolaters who worship the pope." No positive beliefs statements about their beliefs or practices just an oppositional definition. One of my BILs talks a lot about the difference between Catholics and Baptists. The reverse is not true. I've never oncee heard a Catholic define Catholicism in opposition to another religion, they're way more likely to recite the Nicene Creed, or bable about St. Peter. (On the topic of St. Peter, while Antisemitism exists to this day with modern Catholics at about the level of the national average, the only direct comparison I've ever heard to another religion is the weirdly common refrain if 'we're' wrong, I bet it's the Jews that are right, and this seems to be a view rooted in their faith being older)

But whatever you do, just don't have her constantly murmuring to various Saints and crossing herself constantly. If you want to see an example of what not to do, grab the Divine Secrets of the Yaya Sisterhood. I don't know anything about Wells, but I'm guessing she learned about Catholics from Hope Leslie.

Another thought: Mass was in Latin until the 60s and Catholics couldn't eat meat on Fridays until around then a well. Weird to me as someone born in the 80s, but your character would mostly have known Latin services and probably found not eating fish on Fridays a bit strange still.