Why were virgins gathered a second time"
Torah Temimah citing Megilah 13a: Mordechai counseled him that a woman envies only a marital rival [so this will induce her to reveal]. Even so, "Esther did not tell her origin" (20).
Vilna Gaon: They gathered also the remaining Besulos to send them home. This shows Esther's great virtue, like we will explain.
Malbim: [Achashverosh thought that] perhaps she fears to tell her origin, for it is a lowly nation, and he will expel her. He gathered all the Besulos that he did not have Bi'ah with them and sent them home, lest she worry lest the king pick another.
Why does it say "u'Mordechai Yoshev b'Sha'ar ha'Melech"?
Vilna Gaon: He gave to all [the Medinos] tax exemptions and gifts. For Mordechai, a Yehudi, even so he must do something great for him 1 . The king made that he will sit b'Sha'ar ha'Melech, i.e. among the king's officers.
Malbim: Mordechai was elevated to sit in Beis ha'Melech on the chairs of Mishpat like one of the Sarim, because she was found in his house. All the more so they will do honor and grandeur for him if they would know that she is his cousin!
I.e. because he raised Esther, even though the king did not know that they are related. The Vilna Gaon implies that the benefits for the Medinos did not apply to Yehudim. Perhaps seating Mordechai in Sha'ar ha'Melech fixed this; it was a great esteem for all the Yehudim.
