Thank you so much! I'm really pleased that my take on Filius rang true for you, and that the various plot-devices and interpretations of canon happenings seemed to make sense.
You must write more Filius/Severus now, or Filius/anyone.
I'll see what I can do ;-) I must admit, I'd never really considered Filius in a pairing before coming to write this, so it's certainly made me think about the possibilities.