A thoroughly charming story, so nicely written and beautifully characterized. I like snarky Snape and your clever, genuine Filius. The duelling is quite fun, as are the many snappy, revealing exchanges like this one (which parallels so perfectly the one with Minerva):
"Dear me! Apologize? Do you think I should? What ever on earth for?" "For throwing me out a window, perhaps?" Severus suggested. "As I recall, you threw yourself out a window."