For instance, why is Ahasuerus named in verse 1 without the title “king”? Because he was not of royal birth.
Why does the text say that “he sat on his throne”? Because a commoner who becomes king normally makes a point of occupying the traditional throne, whereas Ahasuerus was such an egotist that he had his own throne specially constructed.
Why is Vashti sometimes “Vashti the Queen” (e.g. 1:9) and at other times “Queen Vashti” (e.g. 1:12)? It depends on how and when she became queen. She was of royal blood, the granddaughter of Nebuchadnezzar, whilst Ahasuerus was a commoner who usurped the Persian throne.
To Vashti, her status was always inherently queenly (“Vashti the queen”); to the king she was queen (“Queen Vashti”) only by reason of having become his wife. Her yichus made no difference to him: he was the king, and his word was law.
(One is reminded of the rebbetzin who said to her husband, “You think I’m the rebbetzin because you’re the rabbi? The opposite is true – you’re the rabbi because I’m the rebbetzin!”)