For now, bonuses for other playthroughs are the level 2 curses, and some items you can start out with. I would like to do more eventually though. Same with the mirror stuff, I want to write more for that at some point
The Demon Blood is for making the Demon Werewolf transformation permanent. If you have the werewolf transformation from the Unholy Hollows and then use the demon blood you get a more powerful version of the transformation.