Also, if she's been living in France for a while, presumably she's been to church a few times. Maybe she does it automatically, too. I've seen people do that a lot. At the Lutheran services I've been to in Finland, you do communion halfway through and anyone can go. If she wants to fit in, she'll do everything she can think of and beyond. In that case, why shouldn't she make the sign of the cross upon entering. Everyone's Catholicism is different, but here's what I got from my mom's side (and we'd go to services every Sunday), plus some of her relatives were really religious. For the record I'm French, but please, don't take this to apply to all Catholics. This is merely what I can remember off the top of my head in my family. Feel free to ask more. I can look into it.
1. My aunts (born between 1910-1950) would do the sign of the cross when talking of the dead, their deceased husbands for instance. Some wouldn't even discuss The Act Between Husband And Wife in church, too sacred. Some might make the sign of the cross, as someone said earlier, to gain strength, some might do it to make sure God wasn't mad. Though I can't for the life of me imagine those aunts talking about It to me, too prudish. They wouldn't even say "intercourse", it was always circumscribed as "when she lived with him."
2.The more religious variety would make the sign of the cross whenever they saw a church. As a sign of respect towards God (or to ward off evil).
3. I've seen Catholic Gypsies in Hungary make the sign of the cross when they saw something scary, men and women alike.
4.Again, going by the relatives, some would expect her to make the sign of the cross upon entering the church, some wouldn't. Though you stated that she was alone in the middle of the night. She could either do it "just in case" or else, she could be doing it out of habit (if she's been to a few services, I could also see her doing it as good luck gesture to her daughter.
5. Re money. I've also seen altar boys standing by the altar with baskets, basically implying that if you don't throw some coins in there (though a note would be preferable), you had no business going up to the cross touching it on Easter Sunday. Not saying all churches are like that, just this particular one.
6. Confession is
very important in Catholicism. As my mom put it, "it's basically psychological, like therapy." My Catholic relatives had the following guidelines for themselves: you decide when you go to confession (i.e. if you haven't been to church on Sunday, that's a sin), but you decide for yourself. But if you haven't been to communion once, you can't just go again the following Sunday, you'd have to go to confession first.
Better-versed Catholics and confirm or deny this but if they're getting married, the issue of communion should come up, in that the priest will give it. I might be wrong here though.
Oh yeah, make sure the wedding's not on a Friday, bad taste and bad luck because that was the day Jesus died (again, this is according to my relatives. I'm just putting this in here to give you an idea of how the Catholics I know see it). Also, you can't really serve meat on Friday, it's strictly fish. And it can't be on Ash Wednesday either, you're supposed to fast then.
I really love the idea of the holy water though, don't take it out
And Swedish immigrants in France in 1881, you've hit on another passion of mine. I'm dying to find out what on earth you're writing about