It's weird how much you catch when you're doing a re-write.  As the author you see things in a certain way; as an author of fanfiction, you can't help but notice the little gaps here and there.

JKR didn't have Myrtle help because it wasn't in line with her plot: her help would have made things too easy for Harry & Co.  Since Harry was trapped in this case, I could make better use of her because she could only help with info, moving the characters forward just the tiniest little bit.

-K