Mm, it's a moving target. I mean, there are a lot of fixed facts--I admit I take a strict view as far as The Epilogue--you gotta comply with the big E if you're going for full canon compliance. But there are a lot of squishy places too, especially in characterization.
I agree with Epilogue compliance if you truly want to be canon compliant, as much as it pains me to say. My niece can write a better epilogue and she's 10, but whatever. The squishy places in characterization are one of the reasons I got into fanfic. There were so many places to take the characters.