[1] The Haftarah is read from Neviim and not Kesuvim because the decree was also on Kesuvim. Another reason is because Neviim has topics similar to the Parshiyos of the Torah in contrast to Kesuvim (Levush, Orach Chaim 284:1).

[2] Tosafos Yom Tov, Megilla 3:4 s.v. לכסדרן. Even though the decree became nullified, the Minhag didn’t. 

[3] Shacharis Shel Shabbos, s.v. ולכן.

[4] Levush, Orach Chaim, 284:1.

[5] It is unclear whether the Maftir Aliya was intended to be one of the seven required Aliyos or to be an additional Aliya (Megillah 23a). We accommodate both opinions (See Shulchan Aruch, Orach Chaim 282:4). On Shabbos and Yom Tov we are allowed to add as many Aliyos as we want. We therefore read Maftir as the eighth Aliya, allowing us to follow both opinions. Even if Maftir is meant to be one of the seven required Aliyos, we may add on to those Aliyos. Thus, by reading Maftir as the eighth Aliya, we are keeping in accord with both opinions. The custom of repeating Pesukim developed in the post-Talmudic era, around the year 500. In Talmudic times, they wouldn’t complete the weekly Parsha in seven Aliyos. They would arrange to leave some Pesukim to be read for Maftir and conclude the weekly Parsha, for the first time, with the Maftir Aliya. This change was a result of the Rabbonon Savoroi, sages from the period immediately following the Talmudic era, who instituted that the Kaddish which is said after reading the Torah should be said prior to Maftir. (This was intended to emphasize that the Maftir is not one of the seven Aliyos, but rather an eighth.) For this reason, the weekly Parsha must be concluded before Maftir, since it would be improper to recite a Kaddish in middle of a Torah portion. Thus, the Parsha is concluded, Kaddish is recited, and only then are the last few Pesukim repeated as Maftir (Tosafos to Megilla 23a, s.v. כיון).

[6] Maftir is the Aliya which is given to the person who will recite the Haftarah (or the one who will recite the blessings for the Haftarah). Since the Haftarah is culled from Neviim, it would seem disrespectful to the Torah if someone was called to read only from Neviim, thereby indicating Neviim to be of equal importance as the Torah (Shulchan Aruch Harav 282:10). To ensure the proper respect for the Torah, the rabbis instituted that the one reciting the Haftarah should also receive the last Aliya which is read from the Torah (Megilla 23a). This Aliya is called מפטיר, the verb form of the word הפטרה, since its function is to enable the Haftarah to be read.

[7] Levush, Orach Chaim, 284:1. Mishna Brura 284:2.