The Rambam himself, in the same law (9:6), says that it's permitted to free a slave for the purpose of fulfilling a commandment. Since there are cases that freeing a slave is permitted, the description of how to free a slave can be used for the permitted cases.
ומותר לשחררו לדבר מצוה אפילו למצוה של דבריהם כגון שלא היו עשרה בבית הכנסת ה"ז משחרר עבדו ומשלים בו המנין וכן כל כיוצא בזה וכן שפחה שנוהגין בה העם מנהג הפקר והרי היא מכשול לחוטאים כופין את רבה ומשחררה כדי שתנשא ויסור המכשול וכן כל כיוצא בזה
It is permitted to free a slave for the sake of a mitzvah, even a mitzvah of Rabbinic origin - e.g., if ten free men were not present in the synagogue, a person may free his slave to complete the quorum. Similar laws apply in all analogous situations. Similarly, if people are treating a person's maid-servant in a licentious manner and she presents a stumbling block to sinful people, we compel her master to free her so that she will marry and thus remove the stumbling block. The same principles apply in all analogous situations.
Regarding the other questions:
Is it common for the Mishneh Torah to outline how to do something incorrectly?
I think that describing forbidden actions is common. For example, the different types of forbidden magic in Avodat Kochavim 11. Not entirely analogous, but each type of forbidden magic is described for about as long as each way to free a slave.
Is it common for the Mishneh Torah to describe an action as assur, when it is violating a positive commandment, not a negative one?
I don't think it's common. But there is such a thing as a prohibition coming from a positive commandment. Many of them have corresponding negative commandments (e.g. to rest on holidays but also not to do work on holidays). But here is an example of something that the Rambam says is forbidden exclusively from a positive commandment (Issurey Bi'a 17:13):
מצות עשה על כהן גדול שישא נערה בתולה ומשתבגור תאסר עליו והוא אשה בבתוליה יקח אשה לא קטנה בבתוליה ולא בוגרת
It is a positive commandment for a High Priest to marry a virgin maiden. When she reaches the age of maturity, she becomes forbidden to him, as [Leviticus 21:13] states: "He shall marry a virgin woman." "Woman" implies that she is not a minor. "Virgin" implies that she has not reached maturity.