Why Don't We Mention Chanukah in Al Hamichyah?

If you'll notice, whenever we bentch after bread during the days of Chanukah, we say the "Al Hanisim" prayer before the conclusion of the second brachah. When saying an "Al Hamichyah (or "Brachah Me'ein Shalosh"), we do not mention Chanukah as we do with all the other Yomim Tovim. Why?
In Shulchan Aruch (שו"ע או"ח סי' ר"ח סי"ב) the Mechaber writes the above as the halachah without explaining why. The Mishnah Berurah there (ס"ק נ"ט) brings in the name of the Gr"a that being that mentioning the day even by regular bentching is only a minhag, here where the minhag was never instituted, that's how it remains.
But, one can wonder, why did the minhag in fact never start?
The Mahara'm of Rothenburg explains - mentioning the day is only in context: during bencthing, when we praise Hashem for the good of the land, it follows that we mention His praises for the joyous day that we are celebrating; in the Brachah Me'ein Shalosh where there is no liturgy of praises, we omit it.
תשובות מהר"ם מרוטנברג (דפוס פראג) סי' ע
ועי' שו"ת יביע אומר ח"ג - או"ח סי' ל"ו באריכות שמביא כל השיטות בזה ודן בהם

Nun, Gimmel, Hei, Shin

A certain person authored a book on Jewish customs and laws around the year. He sent the book to the Rebbe in 1954 for a haskamah. The Rebbe wrote him back that he doesn't usually give haskamos, but indulged in leaving comments on various issues throughout the book. Apparently, the author had given certain interpretations of the נ, ג, ה, ש that is displayed on the Chanukah dreidel. The Rebbe wrote to him "Do not mix the explanation for נ ג ה ש that is brought in the writings of great sages of old with English and Yiddish acronyms."
What is indeed the explanation that "the great sages of old" gave?
The classic "נס גדול היה שם" is brought elsewhere by the Rebbe (לקו"ש חי"ב ע' 33 - here) as a "saying" or "lore."
R. Shapiro in his famous work "B'nei Yissaschar" explains the letters of the dreidel in great detail, stating that they are a reference to the four exiles and moshiach who will redeem us from them all (נגהש is the same gematriah of משיח).
אגרות קודש ח"ט ע' סו
בני יששכר כסלו טבת מאמר ב' אות כ"ה
See here

Will Chanuka be Around When Moshiach Comes?

There is a famous quote from the Midrash that "All holidays will expire when moshiach comes except for Chanukah and Purim." The common reference for this is מדרש משלי ט, ב.
Taking a look in the source, one will find that the Midrash only mentions Purim (bringing the passuk from the Megillah "וימי הפורים האלו לא יעברו מתוך היהודים" as proof). Where did this common extension to Chanukah come from?
In his mystical work "Maggid Meisharim," the Beis Yosef writes (פ' ויקהל) "Friday night, 26 Adar II, 1546: ...this is what our Rabbis meant when they said that Chanukah and Purim will never cease..."
Apparently, the angelic Maggid knew, or had a different version of, what the Rabbis said.
Although this may seem odd, Rabbi Yerucham Leiner, following in the footsteps of R. Dovid Luria (famous defender of the authenticity of the Zohar and other Kabbilistic works) goes to great lengths to prove that this is indeed what transpired on that Friday night between the Maggid and the Beis Yosef...
מאמר "זוהר הרקיע" (לר' ליינער, בסוף סםר "קדמות ספר הזוהר לרד"ל) סוף סי' י"ד
ועי' עוד בשו"ת ציץ אליעזר חי"ג סי' ע
For the Beis Yosef's writings, see here
For Rabbi Leiner's writings, see here