Oh, that's so nice! I love Filius and this is a hearty dose of him indeed. So much about your depiction of him fits perfectly with canon. I'm sure he'd be a magical musician, with an odd and original instrument; that he'd have an affection for overstuffed furniture and pink stripes; and that he'd take an alienated young Snape under his wing - and eventually, his covers.
But even more awesome is the writing in this, and the plot that doesn't scant the "interstitial years" or the ending, but rather, gives them the weight and focus to build on the detailed beginning. Now that's something seldom accomplished when we try to embroider our fandom themes upon the unwieldy length of canon.
And your ending is a stunning capstone to this story's multiple arcs! I love that Snape's Kings Cross is a library, I was hopeful that Filius would be there for him. But then, your working-in of Flitwick's dark deaths-edge charm and the song's lyrics! This was genius! GENIUS! Adorable, surprising, and utterly clever genius, just like Filius himself. You must write more Filius/Severus now, or Filius/anyone. I'm more in love with Filius than ever. *brings you both liquorice cakes*