Why does it say "u'Mordechai Yatza"?
Malbim: This teaches the virtue of Mordechai. He did not accept for himself any grandeur until he was sure that there was light and salvation for his downtrodden brothers. Only then he went out in royal dress.
What is "Levush Malchus"?
Vilna Gaon: It is a Talis for the Mitzvah (Zohar).
What is "Tachrich Butz"?
Rashi: A linen Talis proper to cover himself with it.
Vilna Gaon: They are straps that wrap (Tefilin on the body).
What is the significance of the five garments?
Megilah 16b: "[Yosef] gave to Binyamin
Was the Simchah even for Goyim in Shushan?
Malbim: Yes 1 - "bi'Rvos Tzadikim Yismach ha'Am" (Mishlei 29:2).
I.e. for most Amim. Clearly, Amalek and other haters of Yisrael were sad. The Jews killed 800 of their enemies in Shushan (9:12, 15)! (PF)