If it's a spoiler, it's a good one--I was afraid that you might kill Sirius anyway. You don't seem like the kind of HORRIBLE person who would do something like that, but who knows? You've done a most interesting job of borrowing (hijacking! ^^) the same events from canon but having the characters react decidedly differently. Like Harry's finding out about Filch being a squib--in canon he's all apologetic and STILL makes another enemy for life, but in this he's delightfully Slytherin about it. There aren't enough good guys these days with morally ambiguous tactics--I love it!

I think what bothered me the most about Sirius dying in canon (apart from the "Sirius was my favorite character" thing) was that his life was so pointless. He had no great ambition (that we ever find out about), and unless you read slash into he had no great Love either. Most of his life was being miserable in the Black household before he was old enough to leave for Hogwarts, being in Azkaban, or being on the run--and then dying in the most pointless way possible. The best years of his life were in HIGH SCHOOL, and that is indescribably sad.

In short, even if you did decide to stick with canon and kill him off, I wouldn't mind so much because you made his life worth having lived.