I think you've got Brynhild exactly right here, or at least exactly as I've always perceived her in Sigune's stories: brave but intensely pragmatic, an unsentimental woman who shows her devotion in her willingness to do anything for Severus. Best of all, she saves him from the worst of his self-destructive impulses, as when she urges him to forget about Harry and just focus on keeping himself alive. Severus deserves to have somebody who has his best interests at heart, for once in his life.
The whole thing is really well done, and I've enjoyed it a great deal. I think the highest compliment I can pay you is to say that I "saw" the whole story in Sigune's visual style, which says to me that you captured the flavor of her characters very well indeed.