Frank, I'm not well versed on any of the McKees (and other various spellings of the name)in any depth. From your question, I assume you have Bobby Moss' books dealing with the various battle participants? As for Samuel Mackie, I do have a bit more on him from a book I have, "Historical Sketches, Presbyterian Churches and Early Settlers in Northeast Georgia," Groves Harrison Cartledge. Mackie went to GA after the war. If you don't have that book, or can't obtain it locally, let me know. I'll scan for you.
I know very little of the McKays of St. John's Parish who were with Francis Marion, although I became a student of history early in life due to having been born and raised there among the Marion family and the descendants of other such past notables of there. Give me a day or two and I'll look into my Berkeley County files to see if I have anything on those SC Lowcountry McKays.