What is the meaning of "Misyahadim"?
Rashi: They convert.
Vilna Gaon: They made themselves [like] Jews, but 'Yisrael did not accept converts in the days of Mordechai' (Yevamos 24b; we suspect that they do not convert Lishmah). Also, they converted only due to fear.
Why does it mention "Medinah u'Medinah" and "Ir va'Ir [Mekom Asher Devar ha'Melech v'Daso Magi'also"?
Vilna Gaon: "Medinah" is where the Achashdarpanim dwell. They received Devar ha'Melech itself. The small cities received only Daso.
Why does it say "Devar ha'Melech v'Daso"?
Vilna Gaon: Refer to 8:17:2:1.
Malbim: Here, the exposed content of the letter clearly explained what to be ready for. Even the individual cities knew, unlike Haman's letters.