The emphasis of the Rambam in your citation is not about being in a position of communal authority. It is about being the head of the community, meaning the one who determines policy and direction just like the King. A convert is permitted to act as an authority within the chain of command.

The most obvious example comes from Pirkei Avot 5:21-22 which quote as Torah *authorities*, [Ben Bag-Bag][1] and [Ben He-He][2]. They were both Tannaim of the first century and converts to Judaism. Their true names were concealed because of Roman persecution against Jewish converts at the time. Both their names hint to the letter 'Heh' which is a sign of the 'Heh' added to Avraham and Sarah's names. They were both *Ben Avraham*.

Ben Bag-Bag was a student of Rabbi Hillel. In the case of Ben Heh-Heh, he is the authority quoted for Hilchot Talmud Torah in regard to when to begin each type of study, *five for Chumash, ten for Mishnah, fifteen for Gemara, etc*.

In regard to your examples for women, a Rebbetzin is not an appointed position. It is a consequence of her marriage to the community Rav. A mikvah lady is under the direction of the supervising Rav. The question of a Menahelet of a womens yeshivah is dependent upon what her authority is. If she is the final word in making decisions for the yeshiva, it would seem inappropriate. But if she consults with others in committee, it would be fine.

Leadership has many different levels in the chain of command. Both the convert and women are the idea of vessels (*keilim*) which primarily receive influence. Like water from the tap filling the pitcher. But once the vessel is filled, it conveys the water from the tap to the mouth of the one who is thirsting. The most important criteria to being an effective vessel is that it be empty in order to receive from the source.


  [1]: https://en.wikipedia.org/wiki/Ben_Bag-Bag
  [2]: https://www.sefaria.org/person/Ben%20He%20He