^ Sorry but I have the attention span of a squirrel and couldn't bear to read all that, so if I repeat something this person said I'm sorry.

I think that DD had a look of triumph because (spoiler alert for book seven) he realized the mistake that Voldy made. DD knew that Harry had to die in the end, but since Voldy took Harry's blood he made sure that Harry was still tied to him, so even if he (Harry) did die, he would still be tied to life with Voldy, so he would live. Confused? Read the Chapter "King's Cross" in the 7th book and it should support my theory.