I'm not that sure about that question myself, it was more of a hunch. It boils down to this:
The book is permeated with references to dwarf and troll legends, even aside from the Koom Valley story. I thought that, especially when Vimes was taking apart the Where's My Cow? in his head, Pratchett was making a point about these stories. Like a children's book their content depends on what is entertaining or what supports the storyteller's intention (in this case stories about cruel dwarfs and stupid trolls). In particular that as long as the reader is satisfied their mutability is limitless. For example in the dwarf legend that depicts troll as having stolen life. When Bloodaxe speaks through the cube we hear the original version, where trolls are given in life in gratitude. Two opposite situations really, and the first hs simply been formed around the dwarf perspective of Koom Valley. Maybe the link is tenuous, but I thought that particularly during the scene where Vimes unravels the Koom Valley myth while reading, well... shouting, to his son attempted to connect the two in order to make a point.

I agree with you about Sally. She was the only new character that didn't really work.

Originally put forth by Hsing:
I'm not quite sure what you mean. Do you mean the story as a whole or Sally?