Rebbi Yehuda and Rebbi Shimon don't allow us to completely believe a woman's testimony that a girl has reached halachic maturity by growing 2 Sa'aros. You dealt with the issue from the persepctive of a woman's ne'emanus to say testimony. However, why don't we look at it from the perspective of "Ed Echad Ne'eman B'Isurin" and say that the woman is believed just as she is believed to tell us the food she cooked is kosher and that she was Tovel from Tumas Nidah? Why is this case different that we cannot believe her completely according to R' Yehuda and R' Shimon?
Ed Echad is not Ne'eman against a Chazakah when it is not b'Yado. Nidah, in contrast, is b'Yadah li'Tevol.