I'm quite found of reading well-written infidelity fiction, where - such stories are seldom to be found - character makes his choices to stay (or leave) not because of being discovered, or being afraid to be discovered, but because of own decision, sort of as a result of sorting out personal priorities and then giving up the pleasant double life, because s/he chooses, not because s/he is forced to choose by some outer force.
P.S. I must confess, that I tend to interpret your story as "Harry knew about Severus infidelity and decided to fight", namely the line about "corpse-still back", seemed to give such impression. I just love stories, that are free to interpret.