Well, that would theoretically work if it weren't for the fact that the king isn't in Whiteland, he's in Volynia.
The hierarchy goes as follows:
1. King Etwine
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2. The dukes (Harrighfield, Weade, Rossric, Athos families)
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3. The respective nobles under them (earls, too many to count)
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4. The nobles that court in the earls' or dukes' courts. (Think knights, ladies, noblemen with no estates)
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5. The commoners, peasants etc.
The king rules the entire country of Borhilon, which is divided into the regions of Whitelands, Forklands, Falkhalls and the Iron Hills. He rules from the city of Volynia, which is located on the north side of the Falkhalls, though it is his land.
So while your idea would make sense, it only makes sense if the king was in the Whitelands, which he isn't. The war isn't being fought for the Whitelands purposes either, it is simply an internal war between the King and a Duke, about an iron mine. The king has the right to call other vassals to war, and so does the duke if he has allies within the realm. Since the Whitelands are quite loyal to the king, they answered his call to arms and sent an army to help the king, though the Weade family has no direct quarrel with the Perrighfields.
That said, I suppose it does make sense to reveal that the Perrighfields will be going to war against the Whitelands, as a seperate conflict soon after. The duke is dead, after all.
So in that sense, it would make sense for you to play that girl, but she'd not be fixing the war between king Etwine and lord Perris, but rather the war between duke Perris and duke Gregar, who is currently not duke since he has to be inaugurated still.
Can you still follow? It's quite a mouthful. If you want I can PM you some details on the whole ordeal, and give you a hand with a setting for that character, because I believe it's an idea that will work.