Very well written and entertaining book, though there is one thing about the McGuffin that caught my attention, but which nobody brought up.
[spoiler]So, the McGuffin is a Get-out-of-Hell Free Card. Literally. So, you've gotten out of Hell. That doesn't mean they'll let you into Heaven. If you have a Catholic view of the afterlife, then you may still be okay, as you get to do time in Purgatory until you have balanced the proverbial scales. However, if you're taking a Protestant, Purgatory-free view of the afterlife, then you're boned, because that means if you're lucky your soul will be forced to wander the earth for all eternity until something disperses it. If you're unlucky, your soul will be dispersed into the void.
Worse, if any of the villains going after this card have loved ones, if their loved ones are also in The Business, there's a distinct chance that their loved one is going to Hell too. This means that they will also be separated for all eternity. Even if one goes to Heaven and the other goes to Hell, I'd say it's a safe bet that Heaven wouldn't be that fun if you don't have your main squeeze murderous thug to share it with. And that doesn't even get into kids.
Honestly, the only two DC Villains who I could say would really benefit from this card, off the top of my head, would be Cheshire (because of Arsenal and Lian) or Cluemaster (because of Stephanie Brown).[/spoiler]