Not sure about the rest, but it is true that she was "urged" to go on a diet in order to film a cameo for another SW film. I'm not sure how such an effort might contribute to a heart attack, but I hope the two are not related. We are so used to seeing her as Princess Leia and we forget it's not 1977 any more. At any rate, at last report she was stabilized (moved from emergency room to ICU), so hopefully she's gotten through the worst part.
Maybe she was forced to watch Postcards for the Edge as the in-flight movie and it did her in?