For me, dua is a connection with God and anyone who tries to connect with God will have her prayers answered in some way or another. We can pray for ourselves or we can pray for others and any prayer we make is worth something. The way I understand the afterlife is that the Day of Judgement happens at the same time for all mankind. I believe the people who have died are in a timeless state waiting for the Judgement day. It will feel to them as though they died and arrived to the Day of Judgement when really they might have died thousands of years before someone else who is there at the same time... So this is why I also believe that dua for a person who has passed is important. If someone who has passed continues to have his memory live on in a positive way, surely he or she will get benefit for that.
I'm not sure what Islam says about negative dua though... I link it to the evil eye a little in how I think about it. I know the evil eye is about envy but I think that the negative effects can have more to do with what the person was actually doing like bragging or showing off... It's not Islamic to dress to show your class or wealth, so I imagine it's not Islamic to also own things that show how wealthy you are like a giant house or a high end sports car or other such things. So if a person made others envious, he has actually done some wrong himself.
I know it can't be black or white though because people can be envious of normal things, not only things that are extravagant or rich. A person who does not have a child could be envious of her best friend for having a family and it's not because the person with the family is bragging about it necessarily, but the envy falls on her because of the close relationship. But then again, if they were close, the person envying would know it's not right in this case and it would probably not even count as evil eye. Kind of like acknowledging that it's God's will that one has a family and they don't.
Anyway... I feel a sort of link between dua and evil eye... Evil eye can't be evil eye if a person has some envious feelings that they know they should not have and acknowledge are bad, I think... So evil eye is really just someone having negative feelings toward someone that has brought on these feelings in one way or the other... Not trying to blame the victim here, but there are always things that we can do to avoid others having negative feelings towards us and I think it's important that people do them. I had a person tell me once that they knew they were in the right no matter how I felt about the situation and that that was all they needed. In a way, it's true. If they were right, Allah will know. But on the other hand, those feelings would not have existed in the first place if the situation had been dealt with differently, and Allah will also know that!
I'm rambling so I'll sign off.