I found a copy of a Darby family history and it corroborates what you said about Darling and Elizabeth Simpson's daughter marrying Asa Darius Darby.
It says"Asa Darius Darby...was born January 26, 1826 at Baton Rouge, SC.Married Sarah Ann Simpson January 7, 1845 at Baton Rouge, SC.She was daughter of William Darling and Elizabeth (Humphries) Simpson and was born April 17, 1828 in Baton Rouge....He died October 13, 1906 in Baton Rouge, SC.She died August 17, 1886 in Baton Rouge, SC." (Darby Family History, p. 36).
This Asa Darby is the nephew of Mary Darby who married Charner Humphries.
There is also an 1850 census record showing Darling and Elizabeth Simpson in Chester Co. SC.Daughter Sarah is not with them, which would be the case if she were already married.So I tend to think the Darby Family History is correct and the information that Elizabeth and Darling's daughter married James Humphries in Alabama is incorrect.The Sarah Ann Simpson who married James Humphries may be related to this family somehow, but is probably not the daughter of Darling and Elizabeth.
Of course, more information would help in drawing a firmer conclusion.The Darby history does not provide any direct documentation.Therefore, it is not impossible that it is wrong.