I ran across some information since we last corresponded that may or may not help your search. Given the location of Mahalla, there is still a strong possibility that she is the daughter of one of John Spruell's children. Of the older sons, the 1800 census shows no daughters (females under 16) for either George or Simeon.
I don't have the specific information for Luke who was in Pendleton at the time, but other information suggests that he had a later daughter named Mahalla [Several of us are working on Luke's children, and I can send you the info if you are interested]
On the other hand, the 1800 census shows that William has 4 females living in his household under the age of 10. Many of the published genealogies for William lists his sons, but with an a gap in his children from 1787 to abt 1803 -- very possibly the four females are daughters as yet unidentified --
You will also see that William and Simeon were in Laurens county on various business excursions during the early 1800's. For example, William was in Laurens county 20 Feb 1823 as a purchaser from the estate of William Kirkpatrick.
The circumstantial evidence would point toward William as a possibility due to (a)eliminating the other sons of John Spruell based on early census records (and other info);(b) records that show that William was at times in the Larens county area on business; and (c) had females in his household about the age of your Mahalla,
The younger sons of John Spruell such as Jeptha do not match up well -- mostly due to their age as well as other circumstances...