I don't think the place where Carmichel and her dad are is a place where they are "happy", they keep busy and work, but if I don't remember it wrong, it is a place for people who are not ready to move to what's next. Sounds to me like they are unresolved, people who had been unable to "let go" of what happens to the mortal world, which it's not a bad place to be, since they believe they have purpose still, but it doesn't exactly mean "happy", imo.
"Hi, Harry"
"Murphy, you're back. I just recovered from your death. I love you. Don't leave me again."
"Harry, I have to leave. The world is right again and that means I cant stay. I'm sorry for making you hurt again."
Well, character needs time to grow on his own for a while, I agree on that. But I don't think we have to worry about that storyline, if she comes back, she won't be leaving again. I think we're headed for a scenario where Dresden gets to experience what she did when he was gone, and returned.
Anyway, either way, I fully support Murph and Hendricks buddy cop side story... Hell, I support a Murph-afterlife side story in whichever form.