One error should be corrected to start: Most of the Highland Scots were not wealthy slave owners, but exiles who were forced from their homeland. These people were dirt poor and often worked side by side with the slaves, especially in the Carolinas. Culture was no doubt exchanged and also some intermarriage followed. If these descendants had some common ancestors there would have to be cultural bonding as well.
"Presenting" in the churches of the Highland Scots consisted of a leader who would sing or chant in Gaelic one line of a hymn or Psalm. The congregation would then sing the same line in unison and so on until it was finished. This was done because Gaelic Bibles and hymnbooks were scarce and also many of the people were illiterate in their mother tongue. The presenter would be more often chosen because he could read than because he could sing.
   No doubt that among the slaves illiteracy was also very high and this follow the leader format was also adopted. This does not, of course apply to all Black gospel music, but I think you will find that this is the tie being studied.
                   Slainte,
                        Sandy