I loved the use of ambiguity in this fic. The verse you create here sort of a hodgepodge of The End and canon is so gritty and yet so sad. I also really liked that you didn't just automatically redeem Cas, but made it so that there were consequences for the terrible things he did. And it would make sense that even though Sam is technically the more injured party Dean would be the one to hold a grudge years later. Excellent characterization.

I like the emotions in this.

The only thing that jarred me out of the story was Sam's use of the word "mate". Sam being American he would probably use "buddy" or maybe "dude".

Oh my God! I love it!
good job, really good!