The reason the minhag does follow her is probably precisely because she is the one who physically performs the mitzvah, so it’s hard to find an example.”
That is not necessarily true. For example, many Ashkenazic women who marry a Sephardi, follow HIS family minhag for tevila. So let’s say an Ashkenazic woman was married to an Ashkenazi and then was widowed or divorced. Now, suppose she marries again, to a Sephardi. Though her former minhag in the first marriage might have been to immerse 3 times, it may now be that she immerses 7 times, as per the custom of many Sephardic women. I find that very interesting, as this is clearly HER mitzvah, and she should follow her own mother’s teaching (Toras Emeha) according to what you mentioned, but it is not the case. At least, not in my experience with many such women.