Is there any chance that in the case of the German bearers, the name could have been a misreading of e.g. Kar(o)line? In Kurrentschrift, the K somewhat resembles the D from the normal Latin alphabet, so it might be possible that an American immigration officer misread the name (as it was written down by the immigrant) as Dar(o)line and subsequently anglicised it to Darlene.
Also see:
https://en.wikipedia.org/wiki/Kurrent (in English)
You might also want to compare Sütterlin, even though it would be next to impossible to decipher for an American immigration officer, since it is much further removed from the normal Latin alphabet than Kurrent is:
https://en.wikipedia.org/wiki/S%C3%BCtterlin (in English)
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