I like this! Interesting pairing and great set-up. I would definitely keep the flashback scene. I love all the little details about Azkaban, the Dementors and Dumbledore's letter. Since you asked about mistakes, I picked up just two tiny errors: “Why can’t we plant Mandrakes? Their way better then stupid begonias! 'Their' should be 'They're', 'then' should be 'than'.

Great story :)