I rather like your story, well written enough to grab attention and unique enough to hold it. You steer away from the worst cliches about various supporting characters. I especially like how various adults are forced to reconsider their view of Dumbledore without jumping straight into the fight against him. It does keep to the established tropes of avoiding Dumbledore rather than going offensive against him but it does it well.
I'll have to consider subverting the trope with a story where the muggleborns go after pureblood culture itself and change wizarding Britain by force. Bombing the pureblood manors, nerve-gassing Dumbledore's office, turning the Royal Marines on the Ministry and Gringotts.