I think it almost HAS to go like this: we open on Harry getting dressed on his wedding day. He's in the bathroom, shaving. Looking in the mirror.
I'd love for the narration to include: "Yep, that's me. I bet you're all wondering how I got here."
And then we go through the twelve months that got Harry through the year since the Battle of the Bean. Maybe the wrestling case takes place around the therapy and fallout.
And then, in the final chapter, we catch up to Harry in the bathroom on his wedding day. And that's when Mirror Harry makes his move.
I honestly don't see any other way this book can go.