(a) The Gemara says that a woman who does not marry until 40 will not have children. Rashbam gives reasons, including that nobody will want to marry her, and also they should themselves not wait that long. Answering that benos Tzelafchad were tzidkanios only answers the second reason, not the first: why would men want to marry them if they could not have children?
(b) Also, it appears that women came to Moshe Rabbenu's derasha. How does this fit with other sources on women learning Torah?
(a) As the Rashbam writes, the Bnos Tzelfchad had confidence that Hash-m would perform a miracle for them and that they would have children despite their advanced years. Apparently, the righteousness of these women was so well-known that others would also be willing to marry the Tzidkaniyos and rely on a miracle.
(By the way, the term "miracle" is a very broad one. In the sense in which it is used in this Gemara, it does not mean a supernatural occurrence, but rather an uncommon occurrence. Because the daughters of Tzelafchad witnessed what happened to Yocheved, they new that a "true" miracle is not required, just an uncommon occurrence. That is why they were allowed to "rely on miracles.")
(b) Women are permitted, and even expected, to learn the laws that apply to them.