Greg-

Your comments are correct regarding origins, but the historical use of particular instruments and stylistic elements is equally important. If the use of lining-out and the pentatonic scale in religious music can be found among Gaelic Scots, and similar elements are found in African-American gospel, and it can be demonstrated that there was interaction between these groups, it is reasonable to conclude there is probably a connection.

Your earlier comment about the endurance of certain instruments in Ireland and Scotland after they had fallen out of general use in England is a valid point. To offer a relevant parallel, lining-out of hymns was practiced not only in the American South but also in New England in the 18th century (earlier?).