Yeah you're right about the prepot. What I had in mind first with the "~3s" and "~1.5s" was just to give the tank time to pull and not being too fast, but doing it the way you said is probably more practical than my suggestion.
AFAIK CotE doesn't make your dots stronger. That damage is added if the debuff is there.
What all this really comes down to IMO is the value of having Haunt up or not and the time of MG we get in. There's a shitload of factors to count in if we would sit down and try do the math behind it and I get a headache already by just thinking about it. Eventually I'll try give it a take but meanwhile I'll probably just stick to the SimCraft priority order because there's no other proper theorycrafting out there from what I can see, at least not here.