Why did he appoint overseers in all the Medinos?
Rashi: Every overseer knows the beautiful women in his Medinah.
Malbim: If he will not find a proper girl who wants to marry him, appointees will take girls by force. This dispels his fear lest no one agree to marry him, amidst fear lest he kill also her. They were wise to take new appointees. The rich would not bribe new appointees to overlook their daughters.
Was Achashverosh's search for a beautiful girl like David's?
Torah Temimah citing Megilah 12b: No. "Every sharp one acts wisely" (Mishlei 13:6) refers to David. He sought "a virgin" (Melachim I, 1:2) - only one; therefore everyone eagerly brought his daughter, hoping that she will be picked). "A fool spreads folly" (Mishlei, ibid.) refers to Achashverosh. He gathered all beautiful girls [and will have relations with all of them, but only one will become queen 1 ], so people hid their daughters.
Why does it say "Na'arah" (singular)?
Vilna Gaon: Every appointee will pick one from all the beautiful girls, and afterwards [the best] of those gathered will be handed over to Hegai.
What are "Tamrukeihen"?
Rashi: They are matters that make [the skin] clear, like "u'Morak v'Shutaf" (Vayikra 6:21), i.e. fragrant oil, balms and fragrances that purify and soften the skin.
What is the significance of "v'Nason Tamrukeihen"?
Vilna Gaon: they were given from Beis ha'Melech. If not, one cannot tell who is prettiest - perhaps she looks nicer because she had more cosmetics!
Malbim: The Sarisim gave them. This will make them submissive to the Sarisim, and not become haughty like Vashti. She considered it disgraceful for the king to send for her via the Sarisim.