One thing I find curious is in I think every example of the curse in action the death of the person was nearly instant. How'd she survive long enough to make a deal, and direct her curse? Considering how dangerous she was you would think the objective would be to kill as quickly as possible.
Not always. The person who was swarmed by bees didn't die instantly. And in the case of the woman in the shower, Harry had time to intervene and save her even after the curse had hit.
From what we've seen, the Entropy Curse made it look like Margaret was killed in childbirth -- that's not an instant thing, either. Most likely, it created some complication during the birth that was inevitably, but not instantly, fatal.
We've also seen powerful beings able to mess with time, including powerful Faeries. I think it's in the realm of possibility that Lea was able to subjectively extend the time she had to speak with Margaret, not unlike Lash did to Harry in White Night.
We
also know that Margaret made a bad deal with Lea, which, to me, would indicate that she
didn't have a ton of time to work it out. Bad deals are more likely to happen when you're on the spot, and there's no better time crunch to provide incentive than impending death.
I agree that the
concept of her Death Curse was probably something she thought up before; but probably not as a Death Curse. She probably figured it out as a thaumaturgic ritual, in case she had to go up against Lord Raith (Like how Batman has contingencies for others in the Justice League), then used it as her Death Curse when the moment came and she didn't have time to gather other power and materials.