This is an interesting story, not just PWP.
I have a comment about your note at the top. The situation with Dudley being seen as an angel and Harry being seen as the troublemaker when the reverse is true is not all that uncommon. In fact, I saw it first hand with a gang of "elite" boys that were bullying thugs, although they are better compared to Draco Malfoy than Dudley Dursley. Especially with the protection from consequences.
The situation with Snape and the Headmaster was spot on for how I remember the principal reacting to an abusive teacher that combined most of the undesirable elements of Snape and Umbridge in a teacher.