Polish Jews before WW2 almost exclusively paskened like the Rav. When the mishne brura was introduced to Poland, the Gerrer Rebbe ztzl said, after actually urging his followers to buy it, that after a generation or two, Polish Jews will end up paskening like the mishne brura. In fact, a prominent Gerrer businessman and talmid chochom, R’ Avrohom Noach Klein, published a mishne brura with notes as to the differences with the Rav. The main psak in Orach Chaim has become the mishne brura.
The Mechaber and the Tur, when stating clearly that noshim are mechuyov, were talking in the context of davening shemonei esrei specifically. Most rishonim and achronim hold that tefilah, that is the chiyuv to pray to Hashem, is min hatorah. Those who say that it’s m’derabonon mean the actual tefilah liturgy of today which came from the anshei keveses hagedolah. The Mogen avrohom says thats ince the actual wording is miderabonon, that the chiyuv is not min hatorah, only to say any type of prayer. The others say that the rabonon established the liturgy but established it as the way to be yotzei the torah chiyuv. that means that even though the shemonei esrei is rabonon, the chiyuv to daven shemonei esrei, the actual wording, is min hatorah. and therefore since the torah tefilah is a mitzvas asei shelo hazman gromo, women have a torah chiyuv to daven shemonei esrei. Please do the research like I did and study the original documentation to see what I’ve written.