Yessss big T, stuffing love this movie
have seen it so many times now.
I remember having all the same discussions over doobies (long time ago
) trying to work the same things out, but if you get a director's cut copy of the film (has some extra scenes that basically tell you what is going on) and with extra features like the pages from Ms Sparrow's Philosophy of Time Travel the answers are actually all there. I was a little disappointed because IMO it took away some of the magic for me. I just did a quick google to find somewhere showing the book and found this site
http://www.donniedarko.org.uk/philosphy-of-time-travel/This came up too which made me delete everything I just typed because this tells it better and probably more accurately.
http://www.donniedarko.org.uk/explanation/As for what Sparrow said, I always interpreted that as exactly that. Donnie is basically saving the universe and that doesn't change the fact he still dies alone. Sometimes I think maybe it is that we are only in one of possibly an infinite of universes and to die in one means buggery. Like in the end where it shows a universe (I interpret that as the original universe, and the movie is the tangent universe where Donnie is saving the future of the original) in which Gretchen didn't know who he was. It would seem only his family in that universe would be affected.