I guess I view things far differently from you. I don't find any of these instances a stretch.
The Hungarian Darlene is an early example, I wasn't saying it was the source of the name, but obviously there is enough evidence to me to suggest that Darlene has some sort of history outside the US long before it became popular. Plus, I found a Direlyne from 1580s England. Like I said, there are various other very similar forms that are obviously seem related. Doroline pops up (again, not a stretch at all when it comes to vowel shift in different languages). There is also Darlina, Darlen, Dorlen, Diewerlina, etc.
And even the Hungarian origin is not a stretch especially when you had a large enough number of former Hungarian mercenaries settling in the U.S after the Revolution, mainly in the Carolinas. Plus, there were various other examples I found, not just Hungary, mainly Germany, Netherlands and England. I doubt it is Hungarian in origin, I would suspect an obscure Low German or German origin. There were Germans who lived in what is now Hungary for centuries. The Hungarian examples had Magyar surnames, but there was definitely cultural exchange going on between ethnic Hungarians and Germans.
"In the late 19th and early 20th centuries, it wasn't usual for people to look hundreds of years back into their family trees to find obscure baby names. There is a chance it could have happened, of course, but that could only account for a tiny handful of instances."
As for people looking at old records to use names, I never said that is how Darlene became popular. I would suspect more word of mouth, but your statement seems like a personal observation and not a fact. I don't know if people looked at family records or not back then, but I would suspect it may have been more common than you believe since people kept family bibles. I would assume it was a more common source of baby names than it is now. You also need to take word of mouth into account. They may have known their great-grandmother had such and such a name, but not how to properly spell it, so yes, you would absolutely get corrupted offshoots.
This message was edited 3/4/2019, 11:36 AM