I totally agree about
Emma - my sense is that
Emma just looked familiar as a female name and the spelling was tweaked to meet people's expectations. There can't be an etymological link.
As for the BVM's granny ... ! I can't imagine the sturdily Protestant - indeed, Calvinist! -
Dutch settlers in South
Africa paying any attention to any story about the holy family that they couldn't find in the pages of their Bibles. And why a respectable village lady would give her daughter a Latin name, however suitable its meaning would become, three generations later, when the
Romans were the invading conquerors is quite beyond me. Oh well. Perhaps Emerentia was used by
Dutch Catholics, for the reason you suggest, and stayed in use among Protestants because they liked it!