How did Mordechai find out?
Rashi citing Megilah 13b: Bigsan and Seresh were Tarsiyim, and spoke in Torsi. They did not know that Mordechai was from the Sanhedrin, and knew all 70 languages.
What is the significance of telling the king in the name of Mordechai?
Torah Temimah citing Megilah 15a: Whoever says something in the name of the one who said it, he brings redemption to the world.
Vilna Gaon: She did not take grandeur for herself. Rather, she said so in Mordechai's name, for she understood that a miracle will occur via this.
Malbim: He did not tell her to say it in his name. Normally, she did "Ma'amar Mordechai" (20) - exactly what he said. Here she deviated, and said it in his name.
