A few facts that might help clarify some of these relationships:
A certain Alexander Mackenzie, tailor, and his wife Sarah Mansfield had children baptized between 1787 and 1807 at Marylebone and Holborn: Margaret, Mary, Sarah, Jabez, Hannah, Philip, and Euphemia.
This Alexander appears to be the (half?) brother of Lieut.-Col. Jabez Mackenzie of the Bengal Establishment. They were sons of Lady Mary Mackenzie, daughter of the 3rd Earl of Cromarty. Both took their mother's surname. Alexander's paternity is unclear to me at this time (he seems to have been born several years before his mother's first marriage -- i.e. was illegitimate), but Jabez was Lady Mary's son by a certain Robert Clark(e) whom she married in 1750.
Lady Mary Mackenzie married four times, first to Robert Clarke, then to Thomas Drayton, then to John Ainslie, and finally to Henry Middleton. The latter three were of South Carolina.
The Charleston Morning Post, 5 Jun 1786, records the arrival of "A. M'Kenzie, Esq; son of Lady Mary Middleton by a former marriage".