I took a quick look at the reviews and I think the issue is not WHO the third person is but rather the changes from the accepted history of her.
Anyhow I have a suggestion or two on how to get that muse working...
Write a bunch of brief one line plot points (like "Harry and Daphne kick Ron Weasley's shins after Ron insults Neville for having a toad for a pet". When you think you have enough for a couple of chapters, put them in an order that makes sense and start writing.
Another idea is to come up with the major events and write them down. With enough of them you will have a story outline that can help you when your muse decides to visit Antarctica by way of leaky row boat.