Don’t forget that after Voldemort is defeated Ron and Ginny still have four healthy, at-least-reasonably attractive, and genetically probably pretty fecund older brothers, one of whom is already married.
I would wager that by the time Ron and Ginny became parents in their respective marriages, all the given names of close relatives in the parent/grandparent generation had already been bestowed on their older brothers’ kids. (I absolutely agree with Ranger Jeff that the name “Fred” went to George’s first son, for example.)
And since the Weasleys/Potters are a very close-knit clan, they would find it rather inconvenient to have two first cousins with the same given name. (“Oh dear, little Molly fell off her broomstick and sprained her wrist.” “Charlie’s little Molly?” “No, Ginny’s little Molly.” Help.)
I tend to place it at around the 16th-17th century, at least for the UK and probably elsewhere in Europe, following on the upswing in anti-witchcraft attitudes in Muggle culture. That explains some of the technology differential too: magical communities up till that point had shared a lot of material culture with the Muggle majority but then diverged and lost touch with further developments (hence robes, torches, parchment and quills, etc.)
A brief cultural rapprochement around Victorian/Edwardian times (steam trains, newspapers, cameras) more or less fizzled out, probably due both to the impact of the world wars and the overwhelming dominance of modern technology by electricity, which as we know doesn’t coexist with magic very successfully.
Finally, as for the impoverished Weasleys somehow being sufficiently well off to throw a fancy wedding for Bill and Fleur:
- Remember that after (I think) Book 5 Mr. Weasley got promoted to a more important position at the Ministry, so that probably eased the financial strain a bit. So did Percy’s and more importantly the twins’ becoming financially independent and having their own households, not to mention having left school. We know there are scholarships to Hogwarts for really impecunious kids like the orphaned Tom Riddle, but I think the Weasleys would have considered it a point of pride to pay for their kids’ schooling themselves.
One of the things about the Weasleys, after all, is that they’re a sort of “good aristocracy”, as opposed to the Malfoy-type “bad aristocracy”. The Weasleys and Prewitts (Molly’s family) are both old pure-blood wizarding families, and although they’re not conceited bastards about it like the Malfoys, they take their heritage seriously. For example, it makes them initially a bit snobby about, e.g., owning a joke shop as opposed to “proper” careers in public service, even though they’re in what’s delicately called “reduced circumstances” and can’t really afford to be finicky.
Besides being genuinely nice and generous, the Weasleys still think of themselves as having a noblesse oblige role to help others—look how they welcome Harry and Hermione to stay with them for extended periods despite the extra labor and expense entailed in washing their clothes and supplying their Floo powder and all.
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Just because Fleur’s parents couldn’t take on the bridal family’s usual role of hosting the wedding, owing to security reasons, doesn’t mean that they didn’t insist on helping to pay for it.
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Other clan members (in addition to Bill himself and the twins) might well have rallied round to help defray expenses for this big family shindig, even though Molly and Arthur would have been way too proud to ask them for handouts to assist with ordinary household expenses of feeding/housing/schooling seven kids. Even selfish old Auntie Muriel might have reached into her pocket in addition to lending her tiara, although you can bet she demanded to be fulsomely thanked for it.