@roger,
Ooh tricky.
Especially maddening that your fiance doesn't seem to get why it's so upsetting for you. That does seem to be a big part of the whole thing.
My own instincts run towards "confront and put to bed," but roger does make a great point about not putting things in writing. Of course she could as easily pass on whatever you
say, but then it's hearsay. And I can't imagine that someone who has proven herself to be so tonedeaf hasn't stepped on other people's toes as well, so hearsay may well be taken with a grain of salt.
But, the question is when/ how to do that. You wouldn't want to do it at a family gathering sort of thing (spectacle) and I'm not sure getting together for just this one reason is a good idea. The conversation would be short, not worth getting together for even coffee.
Anyway, sympathy for now.