Af hein hayu b'oso hanes (part II)

Yesterday I touched on the issue of "af hein hayu b'oso hanes". The Rishonim debate whether this means that women caused the nes (by Purim, Esther caused the salvation of klal yisrael) or were included in the nes (obviously they were part of the hatzalah).
There is a machlokes between Ran and Rabeinu Tam why women are obligated in eating three seudos on Shabbos (I hope it does not come as a chiddush that women are obligated to eat seudah shlishis just like men are, meaning washing and bentching, etc.) Is it because of the hekesh of zachor-shamor, which means anyone who is obligated in the lav of shabbos is included in all the mitzvos aseh, or is it because "af hein hayu b'oso hanes" of the mun? Obviously, this application of "af hein" assumes being included in the nes is sufficient, which might be the point of machlokes.
The Rambam paskens that a freed eved is chayav in megillah. At first glance, even an eved still enslaved should be chayav, because an eved has the same status as a woman with respect to mitzvos aseh - why does the Rambam specify "eved m'shuchrar"? Perhaps this issue depends on how to understand "af hein". If "af hein" means simply included in the nes, even an enslaved eved should be included, but if "af hein" means causing the nes, perhaps the chiyuv is limited specifically to women and not an enslaved eved.