Avi K,
You wrote: “I would imagine that her sechar for learning those halachot which she must keep (most of Orech Chaim and Yoreh Deah and all of Choshen Mishpat) would be the same as a man’s as she is obligated to learn them.”
I think that this is incorrect. If her schar for learning the laws that she must keep was the same as a man’s, the Gemara would have answered “she is metzuva v’ose on ???????? ???????????? ?????????.”
Rather, she is obligated to learn those laws for practical purposes, not because of a mitzvah of “Talmud Torah” (her learning would be a hechsher mitzvah and non-mutzuveh kiyum of Talmud Torah). And the special protection provided by Torah study only applies to those with a mitzvah of Talmud Torah.