Interesting, and certainly something I want to read.
But, you say that this should be set way after the heresy. Why? As far as I'm aware, there is something of a blank spot regarding the Death Guard. There was mention in 4thEd Chaos Codex that Mortarion led the Death Guard in the attack on Terra, but that's about it. It doesn't mention whether he was daemon or Primarch. There is mention that he has changed, becoming paler, taller and more gaunt, so perhaps he is a daemon Primarch in an earlier form.