if she had been the one who noticed the problems rather than being shown by a spirit guide in widow's weeds.

She did notice, initially, but kept being dragged back under. Plus, we needed a Madame Exposition, otherwise it would've just been Donna talking to herself, and we'd have had people going "Oh, that's a bit unrealistic, she's the only one out of all these people to notice that the world is weird." And we wouldn't have had Miss Evangelista's WARPED FACE I DON'T LIKE WARPED FACES *hides*