DD referred to his brother a couple of the times in the books, but I'm not sure where and I don't remember much about him. I seem to recall that he's dead, but I'm not sure. Unfortunately, our entire HP series is currently on loan to various friends and family members (except for HBP). I don't think any of the dead characters will come back. Rowling has said, "Dead is dead--even in the wizarding world." I think DD will make an appearance in book 7, now that his portrait is in the Headmaster's office at the school. That's an interesting point you make, about the pieces of his soul having varied value. I haven't given that any thought.
And now, I'm off to bed! See you later folks.
I seem to remember Aberforth having had "an unfortunate incident with some goats" or something, and I believe Dumbledore makes a joke about bartenders, implying that Aberforth is the bartender in the Hog's Head.