Did anyone see the back cover of the book already? We know that the front has Harry & Dumbledore looking at the Pensieve but the back cover has Ron, Hermione, and two others who might be Ginny and some boy (it's hard to tell) and they are staring at the Dark Mark in the sky. It looks great!
As for the mirror, it might be helpful between Ron & Harry or Lupin & Harry. I don't think we'll be seeing Sirius again but I do hope I'm wrong on that count.