[screenshot]
Looking at this screenshot out of context, I see a punctuation problem. When the speaker remains the same in a new paragraph, the preceding line should not end with a quotation mark. I haven't paid attention to the prevalence of this, and it may not be sensible to change if it's consistent.
And the "musn't" point is very interesting indeed! Most of the examples on my cursory search are from dialogue or quotes (plays, various Australian newspapers, and a line from Huckleberry Finn), which implies an informality to its usage.